Add sphinx-autodoc-typehints to sphinx docs generation

[djhoese]

https://pypi.org/project/sphinx-autodoc-typehints/

The Satpy devs would like to start using type hinting, but this would end up having redundant information; type definitions in the docstring and type definitions in the annotations. This sphinx-autodoc-typehints extension would make it so we only need to do the type annotations and in the sphinx docs it will fill into the docstring information.

Main question is: can it handle doing this with some functions defining the docstring types and some doing the annotations or does it error out?