Replace autodoc with autoapi when generating Python docs via cmake

[kontura]

We use autoapi in readthedocs because it can parse code directly (code generated by swig) to access doc strings. The benefit is that we don't need to do a full build of the library to create working libdnf5 Python module which is required by autodoc to get doc strings.

We couldn't use autoapi in our regular cmake builds because it was not packaged in fedora (this was not a problem because regular builds have access to the full working Python module and can use autodoc) but autoapi is present since F41 in python-sphinx-autoapi package.

It would be nice to unify the workflows and simplify code by using only autoapi.
There should be no change in functionality.