[DOCS] Fix data stream docs

[jrodewig]

Updates the data streams documentation based on feedback.

Changes:

• Removes collapsible sections. Snippets are now visible without additional clicks.
• Combines the intro and overview pages
• Replaces several bulleted lists with headings and sections
• Overhauls the index template section of 'Set up a data stream'

Any additional feedback is welcome.

Previews

• Intro: https://elasticsearch_59818.docs-preview.app.elstc.co/guide/en/elasticsearch/reference/master/data-streams.html
• Set up a data stream: https://elasticsearch_59818.docs-preview.app.elstc.co/guide/en/elasticsearch/reference/master/set-up-a-data-stream.html
• Use a data stream: https://elasticsearch_59818.docs-preview.app.elstc.co/guide/en/elasticsearch/reference/master/use-a-data-stream.html

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/Data streams)

[elasticmachine]

Pinging @elastic/es-docs (>docs)

[jtibshirani]

I didn't read the text in detail, but I really like the new structure and layout. It nicely addresses the aspects I found unclear earlier.

[debadair]

I left a few comments/suggestions. It occurs to me that we'll ultimately want to show the UI here, as well. Future fun. :-)

I think that consolidating the overview information helps. I struggled with the same problem with ILM and ended up duplicating some info because the Overview just felt incomplete without it.

[jrodewig]

@elasticmachine update branch

[dakrone]

This looks good to me, thanks!

As a separate aside, how should we document the now newly built-in templates for logs-*-* and metrics-*-* as it relates to the new indexing strategy? Do we have docs for that already?

[jrodewig]

As a separate aside, how should we document the now newly built-in templates for logs-*-* and metrics-*-* as it relates to the new indexing strategy? Do we have docs for that already?

AFAIK we don't have docs for them, but we should. I think most of that content belongs with the index template docs. However, we probably want to update these docs to at least mention the priority in case of a naming collision. In any case, that's probably better suited for a separate PR.

I'll coordinate with @debadair to see who will handle that work. Relates to #57629.