DOC-12443: Reorganize GSI documents

[simon-dew]

Jira issue: DOC-12443

This PR doesn't add any new info but reorganizes the GSI devex docs, as a preliminary to adding GSI vector indexes.

• It moves most of the GSI docs out of learn:services-and-indexes/indexes into a new top-level indexes module. This means that the learn module is no longer split between docs-server and docs-devex, which was confusing.
• It updates page aliases of the GSI docs to include the old locations.
• It updates links to the partials, examples, and images, which have of course also moved along with the GSI pages.
• It updates links to all of the GSI pages throughout the devex docs — not really necessary, because we have the page aliases, but makes things easier for authors in the future.
• It moves the overall indexes concept doc, which I think really does belong in the learn module, back to the docs-server repo.
• It deletes some unused images.

Why is the top-level indexes module called that, rather than say gsi? Because by doing so it revives an old coordinate where some of these pages were stored originally. This enables us to remove some old page aliases, even as we're adding new ones, mindful of the advice that we want to keep page alias proliferation to a minimum.

I'll merge these changes into the capella branch and may have to make some follow-up changes in docs-capella at that time.

Docs preview: Primary and Secondary Index Reference — shouldn't actually look any different from before
Credentials: Preview docs for internal review

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

• DOC-12443: Reorganize GSI documents couchbase/docs-server#3672