HTML docs are generated with Sphinx following
https://samnicholls.net/2016/06/15/how-to-sphinx-readthedocs/

It needs the m2r extension, which isn't available on Debian/Ubuntu.
So you have to pip install sphinx-markdown sphinx m2r
(sphinx-markdown may not be strictly necessary).
It doesn't work to have m2r from pip but sphinx from distro packages;
then sphinx-build has /usr/bin/python3 hardwired in, and it won't
see your pip installed m2r.

To generate the HTML docs:
Make sure you have the m2r module installed, then:
    cd sphinxdoc
    make html
