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.
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