docs: add package doc files (#26047)

PR Close #26047

packages/common/PACKAGE.md

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+Implements basic Angular directives and pipes, such as NgIf, NgForOf, DecimalPipe, and so on. 
+
+The `CommonModule` exports are re-exported by `BrowserModule`, which is included automatically in the root `AppModule` when you create a new app with the CLI `new` command.

packages/common/http/PACKAGE.md

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+Implements an HTTP client API for Angular apps that relies on the `XMLHttpRequest` interface exposed by browsers. 
+
+Includes testability features, typed request and response objects, request and response interception,
+observable APIs, and streamlined error handling.
+
+For usage information, see the [HTTP Client](guide/http) guide.

packages/core/PACKAGE.md

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+Implements Angular's core functionality, low-level services, and utilities.
+
+* Defines the class infrastructure for components, view hierarchies, change detection, rendering, and event handling.
+
+* Defines the decorators that supply metadata and context for Angular constructs. 
+
+* Defines infrastructure for dependency injection (DI), internationalization (i18n), and various testing and debugging facilities.

packages/elements/PACKAGE.md

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+Implements Angular's custom-element API, which enables you to package components as
+[custom elements](https://developer.mozilla.org/en-US/docs/Web/Web_Components/Using_custom_elements).
+
+A custom element extends HTML by allowing you to define a tag whose content is created and controlled by JavaScript code. The browser maintains a `CustomElementRegistry` of defined custom elements (also called Web Components), which maps an instantiable JavaScript class to an HTML tag.
+
+The `createCustomElement()` function provides a bridge from Angular's component interface and change detection functionality to the built-in DOM API. 
+
+For more information, see [Angular Elements Overview](guide/elements).

packages/http/PACKAGE.md

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+Deprecated in favor of `@angular/common/http`.

packages/platform-browser/PACKAGE.md

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+Supports delivery of Angular apps on different supported browsers. 
+
+The `BrowserModule` is included by default in any app created through the CLI, 
+and it re-exports the `CommonModule` and `ApplicationModule` exports, 
+making basic Angular functionality available to the app.
+
+For more information, see [Browser Support](guide/browser-support).

packages/router/PACKAGE.md

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+Implements the Angular Router service , which enables navigation from one view to the next as users perform application tasks.
+
+Defines the `Route` object that maps a URL path to a component, and the `RouterOutlet` directive
+that you use to place a routed view in a template, as well as a complete API for configuring, querying, and controlling the router state.
+
+Import `RouterModule` to use the Router service in your app. 
+For more usage information, see the [Routing and Navigation](guide/router) guide.

packages/service-worker/PACKAGE.md

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+Implements a service worker for Angular apps. 
+Adding a service worker to an Angular app is one of the steps for turning it into a Progressive Web App (also known as a PWA).
+
+At its simplest, a service worker is a script that runs in the web browser and manages caching for an application. 
+Service workers function as a network proxy. They intercept all outgoing HTTP requests made by the application and can choose how to respond to them.
+
+To set up the Angular service worker in your project, use the CLI `add` command.
+```
+ng add @angular/pwa
+```
+
+The command configures your app to use service workers by adding the service-worker package
+and generating the necessary support files.
+
+For more usage information, see the [Service Workers](guide/service-worker-intro) guide.
+

packages/upgrade/PACKAGE.md

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+Supports the upgrade path from AngularJS to Angular, allowing
+components from both systems to be used in the same application.