Link mypy docs from typing.rst

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