gh-96917: link to typing.readthedocs.io from typing.rst (GH-96921)

See the discussion at https://github.com/python/cpython/issues/91533
(cherry picked from commit 5b3a2569f4b4dfb58a8f90a241f9dac1a7ea4bf6)

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>