DOC-12443: Reorganize GSI documents

[simon-dew]

Jira issue: DOC-12443

This PR does three things:

1. Moves the indexes concept doc from docs-devex back to docs-server. This is not a new document; it's been languishing in docs-devex since the original migration. It's not used in Capella. I argue it belongs here, in the learn module.
2. Updates links to the index docs in docs-devex, which have been moved out of learn into their own module. The learn module is now completely self-contained within docs-server.
3. Adds vector search to the indexes concept doc.

Steps 1 and 3 (at least) should be backported to Server 7.6.

Docs preview: Indexes
Credentials: Preview docs for internal review

This PR must be merged at the same time as the following:

• DOC-12443: Reorganize GSI documents couchbaselabs/docs-devex#261

[sarahlwelton]

Would like to clean up some of the language around regular FTS indexes, as it paints a poor picture of the functionality. Also axing the old links so we can maybe eventually phase out all of those redirects in the Search files.