Feature/auto docs

Implementing initial documentation using sphinx auto-documentation.

Notes:

1. On the master branch, a GitLab artifact will be created from the runner
   • You can find such an artifact here: https://git.geomar.de/wind-products/windeval/-/jobs/8751/artifacts/download
   • inside the artifact is a index.html file, open that with a browser to view the documentation
   • the artifact somehow can be used for deployment of the documentation on .gitlab.io
   • I've never done this, and I don't know if it is required to set up gitlab pages on the git.geomer.de server https://about.gitlab.com/product/pages/
2. For the documentation the CONTRIBUTING.md was created and placed into the repo-root
3. The LICENSE file got a ReStructuredText header for integration into the documentation

Create the docs locally with

$ # pip install sphinx recommonmark sphinx_rtd_theme sphinx-autodoc-typehints
$ cd doc/
$ make clean && make html
$ firefox _build/html/index.html & # or whatever browser you prefer