Documentation update and cleanup (mostly metadata)

Common:
- bump the remaining old DTDs to the current version;
- update product names and releases (with few exceptions);
- improve wording to match the community status ("by KDE" instead of
  "for KDE").

Documentation:
- remove "installation" section when available (as for the new convention);
- remove obsolete 'package' entity when defined.

Man pages:
- remove "K Desktop Environment";
- cleanup old option references in man.

Note: the kdepasswd documentation is about the now disabled "Password and
User Account" KCM, still available in plasma-desktop sources but probably
meant to be replaced by user-manager. So the document should be either
moved to plasma-desktop or removed if the modules is removed.