Add CI for automatic mkdocstrings version bumps

[ItsDrike]

One of the primary goals of this project is to improve the packaging and provide a more trustworthy package that won't break on end users. Because of that, the mkdocstrings dependency of this library is set to a fairly strict version range, with the upper bound being the most recently released mkdocstrings version that is verified to be supported.

This approach can be fairly tedious to maintain, as it would require keeping track with mkdocstrings releases and make updates manually. A nicer approach would be to make a CI workflow that would run on a cron timer, checking whether a newer release of mkdocstrings is available, running our unit-tests against that and automatically making at least a PR, but perhaps even a PyPI release.