[3.11] gh-103721: Improve cross-references for generic-alias docs (GH-103838) (#103840)

Cc. @adriangb

The "stub documentation" in `types.rst` does already link to the
in-depth docs in `stdtypes.rst`, but the link isn't obvious for new
users. It deserves to be made more prominent.

- Issue: https://github.com/python/cpython/issues/103721
(cherry picked from commit cef542ca572fdd402ff0e10093a0c1b459e5dcd6)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
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