Link mypy docs from typing.rst

[hauntsaninja]

The mypy documentation is the best documentation there is for the Python type system. We should link users to it.

In the long run, typing.readthedocs.io could be a better home, but that shouldn't stop us from helping users today. In my opinion the most likely outcome is that typing.readthedocs.io ends up importing mypy's docs anyway.

Linked PRs

• gh-103099: Link mypy docs from typing.rst #103100
• [3.11] gh-103099: Link mypy docs from typing.rst (GH-103100) #103126
• [3.10] gh-103099: Link mypy docs from typing.rst (GH-103100) #103127