Set up documentation generator

[CyonixC]

(Lack of) documentation was a serious issue in previous iterations. Thus it would be good to set up a documentation generator, such as Doxygen or Sphinx. Include use instructions as well in the README file.

[CyonixC]

Since there are both C++ files and Python files, a combination of Doxygen for C++ documentation generation, Sphinx for Python code generation and documentation formatting, and Breathe for adapting Doxygen output to Sphinx will be used.

A very brief and kinda sketchy quickstart can be found in this Medium article.