Changes:
* Adds additional examples to the `Search a data stream` section of
`Use a data stream`
* Updates existing search docs to make them aware of data streams
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:
* Adds new 'Reindex with a data stream' section to 'Use a data stream'
* Makes the existing reindex API docs aware of data streams
* Rewrites the reindex glossary definition to include data streams
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
