textproc/py-mkdocstrings-python: New port

This Python handler for the "mkdocstrings" package uses Griffe
(= textproc/py-griffe) to collect documentation from Python source code.

Features:

• Data collection from source code: collection of the object-tree and the docstrings is done thanks to Griffe.

• Support for type annotations: Griffe collects your type annotations and mkdocstrings uses them to display parameter types or return types.

• Recursive documentation of Python objects: just use the module dotted-path as an identifier, and you get the full module docs. You don't need to inject documentation for each class, function, etc.

• Support for documented attributes: attributes (variables) followed by a docstring (triple-quoted string) will be recognized by Griffe in modules, classes and even in init methods.

• Multiple docstring-styles support: common support for Google-style, Numpydoc-style, and Sphinx-style docstrings. See Griffe's documentation on docstrings support.