Set up docs

[ColeDCrawford]

Sphinx?

[rlskoeser]

Could also use mkdocs — seems like it might be a bit easier to get up and running? I'm more familiar with sphinx, but have used mkdocs a bit. I have examples of github actions workflows for building and publishing either of these.

Should be possible to set up a docs coverage check on PRs, too. I figured out a way to do that for sphinx, but looks like there's a coverage plugin for mkdocs as well.

[rlskoeser]

@all-contributors
please add @maltevogl for documentation

[allcontributors]

@rlskoeser

I've put up a pull request to add @maltevogl! 🎉

[rlskoeser]

should we consider this task complete, and add a separate issue to setup readthedocs when we're ready?

[maltevogl]

We have now a readthedocs: https://undate-python.readthedocs.io
and could add the button like this

[rlskoeser]

added badge & link to readme, considering this complete