Automated documentation tools

I thought this would be a nice thing. The solution to this problem uses sphinx, doxygen and breathe, where sphinx is the main documentation tool, doxygen is used to parse the C++ source code to some XML format and breathe is a sphinx extension able to read the XML.

In the current setup a ReStructuredText file containing a trivial doxygen directive is maintained for each header in the `src/` directory. One could probably automate this or use a different pattern altogether. I also think it might make sense to strip down the config files for both doxygen and sphinx a little bit, since they contain lots of default stuff and very lengthy instructions on how to use them.