Separate TOC from the navigation

[dhruvmanila]

Summary

This PR does the following:

1. Separate table of contents from the navigation
2. Group certain pages into a single header (like references for settings, rules, etc.)
3. Remove the "Ruff" header in the navigation similar to uv

For (2), the URL doesn't change. For example, even though the settings page is under "References", the url is still "/ruff/settings" instead of "/ruff/references/settings". Ideally, we'd want to move towards the latter URL but it will require creating redirects. I think we could either (a) keep the changes as it is or (b) postpone (2) to be done when we're restructuring the docs and / or adding versioned documentation. @zanieb would love to hear your thoughts here.

Test Plan

mkdocs serve --strict -f mkdocs.insiders.yml, no warnings on generating the documentation site and validate the pages.

[github-actions]

ruff-ecosystem results

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Formatter (preview)

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