Replace ksokoban entity with skladnik one, to follow rename
To prepare for Qt6/KF6 version of skladnik, so its index.docbook can drop defining the entity itself.
Needs drop of these handbook translations from games/skladnik which still refer to KSokoban, both in text and entity:
- po/sv/docs/skladnik/index.docbook
- po/de/docs/skladnik/index.docbook
Edited by Friedrich W. H. Kossebau