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.
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