Install doxygen.

$ sudo apt-get install doxygen graphviz

Install Python packages below. If you install them on the user's local, you need to include ~/.local/bin to $PATH (to use sphinx-build).

$ sudo python3 -m pip install -r ./requirements.txt

Create Doxygen documents.

$ doxygen

Create Sphinx documents.

$ make html

Done. The HTML files will be on _build/ directory.