Oh damn, can we configure mkdocs to use the former URL? Uppercase doesn't look good in an URL :/

I can't find any such configuration unfortunately

You could make a custom redirect using something like this https://github.com/mkdocs/mkdocs-redirects

The redirect works, but at the end of the day it's still a redirect so the URL in your address bar will still be CONTRIBUTING/. The only value it would bring is that the URLs in this readme look a bit nicer, but I wouldn't install a whole plugin just for that.

Ah, I had the impression it would be an url-rewrite instead of a simple redirect. Or actually

During the mkdocs build process, this plugin will create .html files in site_dir for each of the "old" file that redirects to the "new" path

A simpler solution might be to rename CONTRIBUTING.md to contributing.md.

Whilst they suggest to create a file name CONTRIBUTING it does indeed look like lower-case is also supported

Contributing guidelines filenames are not case sensitive

(docs)