doc: support sphinx 3.0.0+
It seems the :function:
directive is much stricter now and requires valid C declaration. The lua functions seem to work okay with :py-function:
. Some of the doxygen generated content was invalid and required various hacks. Now it's working, but I had to omit some symbols.
This should hopefully fix readthedocs issue we're experiencing right now (https://github.com/michaeljones/breathe/issues/495) by switching to sphinx 3.0.0+ in readhthedocs.