Several files in the REST APIs nav section are included using
:leveloffset: tags. This increments headings (h2 -> h3, h3 -> h4, etc.)
in those files and removes the :leveloffset: tags.
Other supporting changes:
* Alphabetizes top-level REST API nav items.
* Change 'indices APIs' heading to 'index APIs.'
* Changes 'Snapshot lifecycle management' heading to sentence case.
Moves the following API sections under the REST APIs navigations:
- API Conventions
- Document APIs
- Search APIs
- Index APIs (previously named Indices APIs)
- cat APIs
- Cluster APIs
Other supporting changes:
- Removes the previous index APIs page under REST APIs. Adds a redirect for the removed page.
- Removes several [partintro] macros so the docs build correctly.
- Changes anchors for pages that become sections of a parent page.
- Adds several redirects for existing pages that become sections of a parent page.
This commit re-applies changes from #44238. Changes from that PR were reverted due to broken links in several repos. This commit adds redirects for those broken links.