Replaces `composable index template` and `composable template` with
`index template` throughout data stream-related docs.
`Composable index template` is only used to contrast with legacy index
templates.
With #58096, data streams now track the timestamp field mapping outside
of the template associated with the stream. This means you can no longer
update the timestamp field mapping using template changes.
This updates the associated data stream docs.
Adds corresponding steps for the get and delete data stream APIs to the
data stream setup tutorial. Also provides some guidance on how to
determine the current write index for a data stream.
Changes:
* Updates 'Data streams' intro page to focus on problem solution and
benefits.
* Adds 'Data streams overview' page to cover conceptual information,
based on existing content in the 'Data streams' intro.
* Adds diagrams for data streams and search/indexing request examples.
* Moves API jump list and API docs to a new 'Data streams APIs' section.
Links to these APIs will be available through tutorials.
* Add xrefs to existing docs for concepts like generation, write index,
and append-only.
Creates a new page for a 'Set up a data stream' tutorial, based on
existing content in 'Data streams'.
Also adds tutorials for:
* Configuring an ILM policy for a data stream
* Indexing documents to a data stream
* Searching a data stream
* Manually rolling over a data stream
James Rodewig