Prepare a Python virtualenv and install all dependencies:

$ virtualenv venv
$ . venv/bin/activate
$ pip install -r requirements.txt

When returning to an existing environment, just do:

$ virtualenv venv
$ . venv/bin/activate

With this, build simply with:

$ PATH_L4RE_TAG_FILE=/path/to/doxygens/l4re.tag make html

Providing the TAG file is optional.

The result can be found in _build/html/index.html.

reStructuredText Primer: https://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html