[3.12] gh-128349: Use `.. data::` instead of `.. class::` for pre-defined decimal `Context` objects (GH-128379) (#128402)

gh-128349: Use `.. data::` instead of `.. class::` for pre-defined decimal `Context` objects (GH-128379)
(cherry picked from commit 60c65184695a3eab766b3bc26fc99f695deb998f)

Co-authored-by: abkmystery <36216019+abkmystery@users.noreply.github.com>