Some improvements to the core module docs

[ciscorn]

• [Sphinx] Add a Sphinx transform to conf.py to convert the first paragraphs into the module titles. (See FIg. 1 & 2)
  • The titles will look similar to those of v5.3.x docs.
  • It also removes refspecific=True from xref nodes so that Sphinx can resolve references to data attributes such as _typing.ReadableBuffer. (See Fig. 1)
• [stubs] Make shared-bindings/_typing/__init__.pyi for the types related to the C-level protocols. We shouldn't use _typeshed as it is typeshed's internal utility.
• [stubs] Add make check-stubs command which validate the generated stubs with mypy.
• [stubs] Fix some type hints.

Fig. 1:

FIg. 2:

[jepler]

I did not review all the docstring changes

[jepler]

Should we add "make check-stubs' to our CI workflow?