Docs: fix most of the broken links in translations that linklint found.

Docs: fix most of the broken links in translations that linklint found.

In most cases, I simply did the same changes as in the English version, so
that the translated versions are the smae as the English version.
In particular, the issues are:

-) Lot of @subsubsections -> @unnumberedsubsubsec, which were done in
   master, but no in the languages. There are lots more to be done, but
   these cause broken links.
-) Gregorian_ligature_engraver -> Vaticana_ligature_engraver
-) "Fret diagrams" node -> "Fret diagram markups"
-) Engraver_group does not have an IR section, so @rinternals -> @code
-) FingerEvent -> Fingering Event
-) @rlsr{Figured Bass} -> @rlsr{Chords}
-) fontSize, font-size does not have an IR section -> remove from IR links
-) MultiMeasureRestMusicGroup, VocalName do not exist in IR -> remove IR link
-) Removed @untranslated sections, which no longer appear in the English
   docs ("Guitar tablatures", "Half-holes")
-) added new @untranslated sections for news sections in English docs
   ("Indicating harmonics and dampened nodes", "Fingerings")
-) Fix IR link "Engravers" -> "Engravers and Performers", similar for
   "Engravers and performers" (case IS relevant in @rinternals!)
-) text-balloon-interface -> balloon-interface
-) broken link to "Octave check" in fr (was "Octavecheck")