Fwd port: Adding .St macro from groff_mdoc package (Standards Names)

forward port of adae2b730c13db9cbfb7728915b3320fd1b1bab5
from old kde-runtime repo

Original commit text:
Adding .St macro from groff_mdoc package (Standards Names)

Adding the .St macro which can display the formal long names
for abbreviated standard identifiers.
Fixes another case of bug #105765