Properly integrate doxygen into sphinx on readthedocs

[bgoglin]

We have documentation on https://hwloc.readthedocs.io/
I broke a while ago because of readthedocs changes, I managed to implement a quick and dirty way to hide doxygen behind sphinx in 2f9da1e

The proper solution (and possibly nicer) is to use things like breathe, exhale or doxysphinx to actually integrate doxygen in sphinx, but that's a lot of work.

Somehow, readthedocs manages to put its own stuff on top of our doxygen pages, including the bottom-left menu for switching between versions. The only issue I found is that the warning about "latest version not being stable" is hidden behind the doxygen title bar.

By the way, we don't use autotools at all on readthedocs, hence we have to duplicate the fig2dev command to generate diagram.png. Might be better to just prebuild diagram.{png,eps} as we do for lstopo outputs since we pretty much never modify diagram.fig.

Ref #160

[trivialfis]

Did some tests with breathe when I was looking into #735 to prevent document duplication. The API reference is not too difficult, not sure about other parts of the document.