docs improvements

[veenstrajelmer]

Todo:

• Fix docs #129
• Add Waterwebservices versus ddlpy example #186
• 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)
• docs: maybe move to myst-nb (extension myst_nb) instead of nbsphinx, this also removes the pandoc dependency but does generate quite some warnings during doc building