[DOC:] New content and new page: Discover main page and multiple data sources (#1711)

* Add new Discover main page

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* New Discover subpage and new content for multi data sources

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* Address tech and doc review comments

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* Copy edit line 129

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* Address editorial review feedback

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* Final edits before merge

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_dashboards/discover/index.md

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+---
+layout: default
+title: Discover
+nav_order: 3
+has_children: true
+---
+
+# Discover 
+
+**Discover** in OpenSearch Dashboards helps you extract insights and get value out of data assets across your organization. You can quickly ingest and query your data, display that data in visualizations and interactive dashboards, and deliver insights to your organization.
+
+<img src="{{site.url}}{{site.baseurl}}/images/discover-index.png" alt="Discover user interface">
+
+## What's next?
+
+- [Running queries in the console]({{site.url}}{{site.baseurl}}/dashboards/run-queries/)
+- [Creating a dashboard with multiple data sources]({{site.url}}{{site.baseurl}}/)

_dashboards/discover/multi-data-sources.md

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+---
+layout: default
+title: Multiple data sources
+parent: Discover
+nav_order: 5
+---
+
+The multiple data sources feature is an experimental feature released in OpenSearch 2.4. It can't be used in a production environment. For updates on the feature’s progress or to leave feedback on improving the feature, see the [OpenSearch Forum discussion](https://forum.opensearch.org/t/feedback-experimental-feature-connect-to-external-data-sources/11144).
+{: .note }
+
+# Multiple data sources
+
+You can add multiple data sources to a single dashboard. OpenSearch Dashboards allows you to dynamically manage data sources, create index patterns based on those data sources, and execute queries against a specific data source and then combine visualizations in one dashboard.
+
+In this tutorial we provide the steps for enabling the `data_source` setting in Dashboards; adding credentials, data source connections, and index patterns; and combining visualizations in a single dashboard.
+
+## Try out the multiple data sources feature in your local environment
+
+This tutorial uses a preconfigured data source and index pattern, and you aren’t required to configure settings. However, you’ll need to enable the `data_source` setting in the configuration file before before getting started with exploring this feature.
+{: .note }
+
+The multiple data sources feature is experimental and can't be deployed into production. You can try it out with a sample data source and a sample index pattern. Before getting started, you must first edit the YAML configuration. The following section provides the steps for enabling the feature.
+
+## Edit the YAML configuration to enable the multiple data sources feature
+
+Dashboards is configured in the cluster settings, and the multiple data sources feature is disabled by default. To enable it, you need to edit the configuration in `opensearch_dashboards.yml` and then restart the cluster.
+
+To enable the feature:
+
+1. Navigate to your Dashboards home directory; for example, in Docker, `/usr/share/opensearch-dashboards`.
+2. Open your local copy of the Dashboards configuration file, `opensearch_dashboards.yml`. If you don't have a copy, get one from GitHub: [`opensearch_dashboards.yml`](https://github.com/opensearch-project/OpenSearch-Dashboards/blob/main/config/opensearch_dashboards.yml).
+3. Set `data_source.enabled: false` to  `data_source.enabled: true` and save the configuration.
+4. Restart the Dashboards container.
+5. Verify the feature configuration settings were created and configured properly by connecting to Dashboards through [http://localhost:5601](http://localhost:5601/) and viewing the **Stack Management** console. **Data Sources `Experimental`** will appear in the sidebar. Alternatively, you can open on [http://localhost:5601/app/management/opensearch-dashboards/dataSources](http://localhost:5601/app/management/opensearch-dashboards/dataSource).
+
+## Create a data source connection
+
+A data source connection specifies the parameters needed to connect to a data source. These parameters form a connection string for the data source. In Dashboards, you can add new data source connections or edit existing connections.
+
+To create a new data source connection:
+
+1. Open Dashboards. If you’re not running the security plugin, go to [`http://localhost:5601`](http://localhost:5601/). If you’re running the security plugin, go to [`https://localhost:5601`](https://localhost:5601/) and log in with the username `admin` and password `admin`.
+
+2. In the Dashboards console, select **Stack Management > Data Sources `Experimental` > Data Sources > Create data source connection**.
+<img src="{{site.url}}{{site.baseurl}}/images/multi-data-sources-1.png" width=500 alt="Data sources user interface">
+
+3. Add information to each field to configure **Connection Details**, **Endpoint**, and **Authentication** to connect to a data source. For this tutorial, the **Endpoint URL** is `http://localhost:5601/app/management/opensearch-dashboards/dataSources`.
+<img src="{{site.url}}{{site.baseurl}}/images/multi-data-sources-2.png" width=500 alt="Create a data source connection user interface">
+
+4. Select  **Create data source connection** to save your settings.
+
+5. Return to the **Data Sources** main page to confirm that the newly created data source is listed under **Data Sources**.
+<img src="{{site.url}}{{site.baseurl}}/images/multi-data-sources-3.png" width=500 alt="Data sources list user interface">
+
+6. (Optional): Select the data source to verify that the settings are configured properly.
+<img src="{{site.url}}{{site.baseurl}}/images/multi-data-sources-4.png" width=500 alt="Data sources settings verification user interface">
+
+## Create  an index pattern
+
+Index patterns allow you to access the OpenSearch data that you want to explore. An index pattern selects the data to use and allows you to define the field properties. Learn how to load your own data and create an index pattern following these steps. This tutorial uses the preconfigured index pattern `opensearch_dashboards_sample_data_ecommerce Default`.
+
+1. In the Dashboards console, select **Index Patterns > Create index pattern**.
+<img src="{{site.url}}{{site.baseurl}}/images/multi-data-sources-5.png" width=500 alt="Index pattern user interface">
+
+2. Select **Use external data source connection**.
+3. Start typing in the Search data sources field to search for the data source you created earlier and then select the data source and **Next step**.
+<img src="{{site.url}}{{site.baseurl}}/images/multi-data-sources-6.png" width=500 alt="Index pattern search user interface">
+
+4. Add an **Index pattern name** to define the index pattern and then select **Next step**.
+<img src="{{site.url}}{{site.baseurl}}/images/multi-data-sources-7.png" width=500 alt="Index pattern define user interface">
+
+5. Select an option for the **Time field** and then choose **Create index pattern**.
+<img src="{{site.url}}{{site.baseurl}}/images/multi-data-sources-8.png" width=500 alt="Index pattern time field user interface">
+
+## Search data
+
+Before you start searching for data, set up the time filter. The sample index pattern used for this tutorial contains time-based data. You can set a time filter that displays only the data within a specified time range, and you can select the time filter to change the time range or select a specific time range in the histogram.
+
+### Use the time filter
+
+1. In the Dashboards console, select **Discover** and confirm the index pattern being used is `opensearch_dashboards_sample_data_ecommerce`.
+2. Select the calendar icon to change the time field. The default is **Last 15 minutes**.
+3. Change the time field to **Last 7 days** and select **Refresh**.
+<img src="{{site.url}}{{site.baseurl}}/images/multi-data-sources-9.png" alt="Time filter user interface">
+
+4. To set the start and end times, select the bar next to the time filter. In the popup, select **Absolute**, **Relative**, or **Now** and then specify the required options.
+<img src="{{site.url}}{{site.baseurl}}/images/multi-data-sources-10.png" width=300 alt="Start and end times user interface">
+
+### Select a time range from the histogram
+
+To select a time range for the histogram, you can do one of the following:
+
+* Select the bar that represents the time range you want to zoom in on.
+* Select the bar and drag to view a specific time range. You must start the selection with the cursor over the background of the chart—the cursor changes to a plus sign when you hover over a valid start point.
+* Select the dropdown and then select an interval.
+
+<img src="{{site.url}}{{site.baseurl}}/images/multi-data-sources-11.jpg" alt="Histogram user interface">
+
+## Create visualizations
+
+Follow these steps to learn how to connect your visualizations in a single dashboard:
+
+1. In the Dashboards console, select **Visualize** > **Create visualization**.
+2. Select the visualization type. For this tutorial, select **Line**.
+3. Choose a source. For this tutorial, select the index pattern `opensearch_dashboards_sample_data_ecommerce`.
+4. Under **Buckets**, select **Add > X-axis**.
+5. In the **Aggregation** field, select **Date Histogram** and then **Update**.
+6. Select **Save** and add the file name. This tutorial uses preconfigured visualizations, so you won’t be able to save your visualization.
+
+## Connect visualizations in a single dashboard
+
+Follow these steps to connect your visualizations in a single dashboard: 
+
+1. In the Dashboards console, select **Dashboard > Create dashboard**. 
+2. Select **Add an existing** and then select the data you want to add.
+3. Select **Save** and add the dashboard name in the **Title field**. This tutorial uses preconfigured dashboards, so you won’t be able to save your dashboard.
+4. Click on the white space left of **Add panels** to view the visualizations in a single dashboard.
+
+Your dashboard might look like this:
+
+<img src="{{site.url}}{{site.baseurl}}/images/multi-data-sources-12.jpg" width=500 alt="Example dashboard using data visualizations from many data sources">
+
+You have now explored the data sources experimental feature. We look forward to your feedback on how we can improve this feature ahead of its release for production use.
+
+## Limitations
+
+The following limitations apply to the OpenSearch 2.4 release of this experimental feature:
+
+* The multiple data sources feature is supported for index-pattern-based visualizations only.
+* The visualization types Time Series Visual Builder (TSVB), Vega and Vega-Lite, and timeline are not supported.
+* External plugins, such as Gantt chart, and non-visualization plugins, such as the developer console, are not supported.
+
+## Related topics
+
+* [OpenSearch 2.4.0 is ready for download](https://opensearch.org/blog/)
+* [OpenSearch Forum](https://forum.opensearch.org/)