Surface sponsors on welcome page and sidebar

[Kludex]

Summary

• Added a Sponsorship section to the welcome page mirroring Starlette's pattern, with FastAPI and Hugging Face logos linking to their sites and a CTA to GitHub Sponsors.
• Added a persistent sponsor block at the bottom of the sidebar (logos + "Become a sponsor" button) via a nav.html override and an extra.css stylesheet.
• Removed the standalone sponsorship.md page (and its nav entry) in favor of linking directly to https://github.com/sponsors/Kludex.
• Inverted the Hugging Face wordmark in dark mode so it stays readable.

Test plan

• mkdocs build --strict passes.
• Local mkdocs serve shows the sponsor section on the welcome page and the sidebar block on every page.
• Sidebar sponsor logos render correctly in both light and dark modes.

AI Disclaimer

This PR was developed with the assistance of either Claude or Codex. I've reviewed and verified the changes.

[github-actions]

📖 Docs preview: https://10bdc460.uvicorn.pages.dev

[codspeed-hq]

Merging this PR will not alter performance

✅ 24 untouched benchmarks

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_{Comparing docs-sponsors-overhaul (828deec) with main (10ddc6d)}