Add type hint stubs and generate documentation from them

I've been wanting to add type hints for the _drgn C extension for
awhile. The main blocker was that there is a large overlap between the
documentation (in docs/api_reference.rst) and the stub file, and I
really didn't want to duplicate the information. Therefore, it was a
requirement that the the documentation could be generated from the stub
file, or vice versa. Unfortunately, none of the existing tools that I
could find supported this very well. So, I bit the bullet and wrote my
own Sphinx extension that uses the stub file as the source of truth (and
subsumes my old autopackage extension and gen_docstrings script).

The stub file is probably incomplete/inaccurate in places, but this
should be a good starting point to improve on.

Closes #22.

Author: osandov

.readthedocs.yml

@@ -1,6 +1,3 @@
 version: 2
 sphinx:
   configuration: docs/conf.py
-python:
-  install:
-    - requirements: docs/requirements.txt