This PR includes documentation for the z3py compatibility API into our general API.
It does so by adding the z3py compatibility API as an external project to download it and then have sphinx document it via the autodoc extension that we already use for our regular python API.
Right now we simply show everything on one page, which should be refactored in the future.
This PR adds documentation to the python class Op, Grammar, Result, and for API enums.
Additionally, documentation for isNull functions in the datatype classes is added for the python API, and a small change in the cpp API documentation is introduced.
