doc: do not use a few deprecated entities

- &koffice; and &kword; can be safely replaced by &calligra; and
  &calligrawords;. The latters have been in KDocTools for a long
  time, for sure from 5.15.
- both &arts; and &DCOP; can be dropped: the glossary items
  do not use them at all anywway. So dropping the entites should help
  with consistency in case the translation uses a different spelling
  then the glossary one.