Add docs for advanced indexing features

[kilfoyle]

This adds documentation to enable and disable advanced indexing features: TSDS and synthetic _source.

@kpollich Thanks for the really excellent draft! I made only very minor changes.

Rel: #151

PREVIEW

[andresrc]

AFAIK, as of 8.8.0 these features are no longer experimental. If this is correct, I'd probably go "Advanced Indexing Features" or something like that and remove the technical preview banner /cc @joshdover @ruflin

Edit: clarified at #227 (comment) . In any case, this comment is not blocking we can always fine tune in follow-up PRs

[ruflin]

I'm approving this change on my end. It is key that we have the page, now we can start to iterate on it with smaller PR's / changes.

[kilfoyle]

@andresrc

AFAIK, as of 8.8.0 these features are no longer experimental...

Coincidentally, I worked on the TSDS docs. The feature is GA. I think all that matters is that the synthetic source docs (in the Elasticsearch guide) are marked with the admonition, and they are:

So I've removed the technical preview banner from this page, and I've changed all instances of experimental to advanced.

[lalit-satapathy]

Approving as the first cut version. We will add any tweaks needed in future docs PR.

[kilfoyle]

Thanks for thorough and super fast reviews!

I'll merge and backport this into 8.8, and I'm happy to help iterate on whatever updates we still need.

[kilfoyle]

💚 All backports created successfully

Status	Branch	Result
✅	8.8

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