documentation/python: initial support for generating Python stubs.

documentation/python: initial support for generating Python stubs.

So far just aiming to produce valid code, skipping docstrings altogether.
Thus -- generating one file per module, and putting code that closely
resembles what's in the documentation, but in a way that is actual
Python, together with importing dependency modules.

To support incomplete or broken annotations, the tyoe_relative template
variable got specialized to type_quoted. In certain cases, such as base
classes for enums or (data / default) values, it's left as *_relative, as
in those cases no quoting is necessary. What isn't handled so far is
quoting forward references for types that were not yet listed in the
output.