build sphinx documentation

1. we use sphinx for the technical documentation of mutwo
2. online documentation is build automatically via readthedocs with every commit (and published here)
3. when new classes or functions are added to mutwo, sphinx apidoc tool has to be run
   1. simply run sphinx-apidoc -M -d 2 -o docs/ mutwo/
   2. this will update the automatic api documentation
4. for locally testing the html documentation, move to 'docs' and run make html
5. the dependencies for building the documentation can be found in 'docs/requirements.txt' (the requirements file doesn't include mutwo itself which obviously has to be installed too for building the documentation. When the documentation is build via readthedocs mutwo will become installed via the readthedocs.yml file)