Add outdated version warning

[danyeaw]

As suggested by @kattni (ty!) during a community call, it is currently easy for users to find a version of our docs that aren't the latest version. This PR adds an outdated mkdocs extension to show a warning with a link to view the latest version. I was trying to go for clear but slightly playful on the message - open to different wording as well.

I tested this locally with:

mike deploy 2025.7.1 latest
mike deploy 2024.12.1 old
mike set-default latest
mike serve

[WebReflection]

Thank you!

[danyeaw]

Thanks @WebReflection for reviewing. I don't have write access to this repo, so you'll have to merge it as well.