Migrate documentation to sphinx + breathe

I would like to migrate the LALSuite documentation away from pure Doxygen to something Sphinx-based that uses breathe to bridge between Doxygen and Sphinx. This would allow significantly more flexibility in the way that the documentation is presented for each package, and the top-level build, and would make it much easier to improve the docs.

I don't expect to have much time to put into this idea, so if anyone is willing to contribute that would be excellent, please reach out to me.