doc: Revamp KCM documentation
This was pretty old already and the KCM received a large overhaul in Plasma 6.0. Added lots of new and hopefully helpful text, further hoping that it won't become outdated too soon.
Submitting for 6.1 due to string freeze.
For your convenience, here's a rendered version as generated with meinproc6
(not sure how to apply the KDE theme):
Edited by Jakob Petsovits