DOC - Enhancements to Sphinx documentation

[trallard]

This PR;

• Fixes the index.md syntax so that the grid is visible; when this was ported, some linter/formatter might have messed up with the markdown syntax. Is there a way to skip this document or markdown files in /docs, and we can format those via pre-commit? Otherwise, this might continue to cause issues with myst/rst formatting in the long run
• Hides in-page table of contents
• I also changed the slug in the PR action to google-grain, but I am unsure if this is correct (need @iindyk to confirm).
• Remove private_members from autoapi

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📚 Documentation preview 📚: https://google-grain--648.org.readthedocs.build/

[iindyk]

ported in #649