[3.12] Link to the glossary for "magic methods" in ``MagicMock`` (GH-111292) (#114002)

Link to the glossary for "magic methods" in ``MagicMock`` (GH-111292)

The MagicMock documentation mentions magic methods several times without
actually pointing to the term in the glossary. This can be helpful for
people to fully understand what those magic methods are.
(cherry picked from commit e97da8677f7bbc6d970e230d334cd646ab662af1)

Co-authored-by: Pierre Equoy <pierre.equoy@canonical.com>