Set up documentation using mkdocs

[stefsmeets]

This PR sets up documentation using mkdocs. You can build the documentation for yourself like this:

pip install -e .[docs]
mkdocs serve

Once this is merged I'm happy to set up documentation on readthedocs under https://fwljanus.readthedocs.io

Closes #29

[timlichtenberg]

So my only point why hesitating with this is the consideration of whether we want to have individual docs for all sub-modules, or whether they should all be incorporated into proteus documentation. I cannot exactly remember anymore whether we agreed this was the best solution or not, we discussed this at one of the last meetings.

[stefsmeets]

This can be solved afterwards using subprojects: https://docs.readthedocs.io/en/stable/subprojects.html

[timlichtenberg]

Awesome, this sounds good to me then!

[stefsmeets]

If I have time tomorrow I will set this up on readthedocs.