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

See the discussion at #91533
(cherry picked from commit 5b3a256)

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

Author: miss-islington

Doc/library/typing.rst

@@ -39,6 +39,11 @@ provides backports of these new features to older versions of Python.
 For a summary of deprecated features and a deprecation timeline, please see
 `Deprecation Timeline of Major Features`_.
 
+.. seealso::
+
+   The documentation at https://typing.readthedocs.io/ serves as useful reference
+   for type system features, useful typing related tools and typing best practices.
+
 
 .. _relevant-peps: