docs improvements

[veenstrajelmer]

Todo:

• jupyter notebook on binder requires manual pip install -e ../../. for some reason. It also requires pip install matplotlib xarray because these are optional dependencies
• document minimal dependency versions and add mindeps pytest?
• update contributing docs
• fix python code coloring in embedded ipynb (it does succesfully color when generating docs locally)
• fix code quality badge (& in url goes wrong): Including a README.md with links that contain &s, "translates them" to & omnilib/sphinx-mdinclude#19