add sphinx docs

as discussed last week during the potsdam meeting (ping @viktoria-wichert)

you can build the docs via

cd docs
pip install -r requirements.txt
make html

The documentation is then available at _build/html/index.html and can be opened with any webbrowser (e.g. firefox).

A PDF can be compiled via latex using

make latexpdf

and can then be found in _build/latex/fee-river-plume-workflow.pdf.

I recommend that you fill the docs/user-manual/index.md with content. If you want to add more files to the user-manual, please use the toctree directive (see it's implementation in docs/index.md). For any other functionality, I recommend to checkout the docs of myst-parser. I also added the sphinx-design extension whose features I find quite useful. But we can also remove that if there is no need for it.