Add PyPI downloads badge to documentation site header #83

Description

Add a PyPI monthly downloads badge to the documentation site (mkdocs.yml or docs/index.md) header, matching the new README badge order.

What to do

1. Open docs/index.md (or equivalent landing page)
2. Add [![Downloads](https://static.pepy.tech/badge/ondine/month)](https://pepy.tech/project/ondine) badge
3. Ensure badge order matches README: PyPI version → Downloads → License → Python → Stars → CI → Docs

Acceptance criteria

• Badge renders correctly on the docs site
• Badge links to pepy.tech/project/ondine