update contributing docs now that mkdocs-material-insiders is free for everyone :)

[tsvikas]

Material for MkDocs Insiders is now free for everyone
https://squidfunk.github.io/mkdocs-material/blog/2025/11/11/insiders-now-free-for-everyone/

You might want to update the docs:
https://docs.astral.sh/ruff/contributing/#mkdocs

and also might want to update relevant build tasks, if any.

[mahiro72]

Hi @tsvikas , I'd like to work on this.

[tsvikas]

Hi @tsvikas , I'd like to work on this.

Great, thanks!