[3.13] decimal docs: specification link and examples (GH-128698) (#142805)

decimal docs: specification link and examples (GH-128698)
(cherry picked from commit 2450be607cfbdb1ab9e4ed7c0fe01d715e406d18)

Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>
Co-authored-by: RUANG (James Roy) <longjinyii@outlook.com>
Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>