Admonition at the top of `CONTRIBUTING.md` badly rendered by mkdocs

### Summary

The admonition here is malformed:

https://docs.astral.sh/ruff/contributing/

![Image](https://github.com/user-attachments/assets/5fa17d8b-05f3-42d7-bcc1-716c9baa5007)

### Version

NA