Use doxygen for code documentation

[leha-bot]

CMake has a built-in support for Doxygen, the powerful documentation generator.
We could add the additional CMake option(BUILD_DOCS "Build Doxygen documentation" OFF) and generate it at CI on demand or dump to some GitHub Pages.

Doxygen also could be befriended with Sphinx, the Python-powered documentation generator with more modern Webpages generator - e.g. see the Kitware's blog post about Doxygen-Sphinx.

[alandefreitas]

Sure. That sounds good. I'm not sure all functions are so well documented but the comments are reasonable and they are in the doxygen format already. It's always better than no documentation anyway.

The github pages is using the just-the-docs theme and it automatically includes all .md files from ./docs in the navigation bar by default. I don't think there's another way to create another link in that navigation bar on the left.

So I'd have to make some tests to see what I can integrate these. Maybe the doxygen documentation could go to a subdirectory of ./docs and I could force a new link to this documentation here:

matplotplusplus/docs/_config.yml

Line 8 in d0ef918

aux_links: