Add documentation

[FlorisCalkoen]

Both for the package and cookbook we still have to write docs. I would suggest to document any function following Google docstring format, which we then assemble into a Sphinx documentation (e.g., pydata layout) and publish on a static page. For the package we can fully automate the process. For the cookbook (examples of how to build/maintain notebooks for this course) we might have to do some research. Also it would be useful if we could attach some figures that illustrate the overall architecture.