CMake: Build documentation

[ZedThree]

Resolves #1937

Pretty bare bones. There's a more complete method described in https://devblogs.microsoft.com/cppblog/clear-functional-c-documentation-with-sphinx-breathe-doxygen-cmake/

Currently we always run doxygen (via subprocess.call in conf.py), a nicer method would be to only run it when needed, which the above link describes how to do.