[REQUEST]: Document "write to streams" feature for Elasticsearch output + integrations

[kpollich]

Description

We recently added functionality to Fleet, Beats, and Integrations to allow writing data to streams.

This feature involves a few steps to opt in, as agents require broader permissions that normal to write to the data streams patterns used by streams. We'd like to document these steps publicly to drive adoption of the streams feature.

Resources

@rdner Provided detailed end-to-end testing instructions that should be suitable as draft content for this docs here: https://github.com/elastic/ingest-dev/issues/5891#issuecomment-3312366758

Collaboration

The product team will provide the initial content.

Point of contact.

Main contact: @rdner @flash1293

Stakeholders: @elastic/streams-program-team

[flash1293]

cc @LucaWintergerst @mdbirnstiehl we should align this with the streams UI documentation and the Elasticsearch-level documentation. At the very least things should be linked up properly

[mdbirnstiehl]

@kpollich I'm finalizing the docs plan for streams right now, I'll factor this in. I'll reach out to the ingest docs team as well to make sure they are aware.