[Docs] Improve API docs (+small tweaks)

[hmellor]

Merge after tlambert03/mkdocs-api-autonav#23 and mkdocs-api-autonav release.

• Removes yaml frontmatter and unused explicit anchor from intel doc
• Removes workaround from mkdocs.yaml and re-enable navigation.indexes to improve the UX of the API docs

Before	After

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[gemini-code-assist]

Code Review

This pull request aims to improve the documentation by tweaking the mkdocs configuration. The changes include standardizing page titles to use Markdown headers and removing an obsolete configuration. However, one of the changes in docs/.nav.yml might have an unintended side effect on the API reference navigation. I've added a comment with a suggestion to address this potential issue.

[DarkLight1337]

The docs LGTM, thanks!