feat(docs): add badges to docs landing page

[Alberto-Codes]

Context

docvet's landing page has a row of badges (PyPI version, Python versions, CI status, codecov, license, docs vetted). Our docs site landing page (docs/index.md) goes straight to prose. Badges give instant credibility signals to drive-by visitors.

Scope

Add badge row to docs/index.md matching the badges already in README.md:

• PyPI version
• Python versions
• CI status
• Coverage
• License
• Ruff
• docs vetted (docvet)

Notes

Can bundle with the branding issue or do independently.