Add basic documentation for Searchable Snapshots REST APIs

[tlrx]

This pull request adds basic documentation for the searchable snapshots REST APIs. The main motivations are to not break downstream projects (see #53871) and to provide a simple example of how to mount a snapshot.

It adds a new "Searchable Snapshots APIs" sub section in the REST APIs section. The "Mount Snapshot API" is the more complete documentation and provides an example of how to create a new index backed by a snapshot.

Those API are experimental and marked as such. I've not seen any mention of the license, except the [testenv="basic"] tags that I copied from other doc.

[elasticmachine]

Pinging @elastic/es-docs (>docs)

[elasticmachine]

Pinging @elastic/es-distributed (:Distributed/Snapshot/Restore)

[DaveCTurner]

@elasticmachine please run elasticsearch-ci/docs

(the build logs are gone, I can't work out how to see the preview)

[DaveCTurner]

Looks OK as far as I can see; I don't understand why I can't see a preview of these docs from CI however, so we may need to follow up once the docs become properly available.

[tlrx]

@elasticmachine update branch

[tlrx]

Thanks David!

I don't understand why I can't see a preview of these docs from CI

I did not know that CI was exposing a preview of the docs. I built the docs locally and ran the docs tests, but there's always a room for breaking doc after that :)