Update/adapt kmag docbooks to kf5

remove entities kappname + package unused in kf5
replace entity kmag with kmagnifier defined in general.entities in kdoctools
bump date + releaseinfo
replace 'for the K Desktop Environment (&kde;)' with 'by the KDE community'
remove empty chapter faq
remove appendix installation - unused in kf5
remove obsolete comments
update screenshot
adapt man page docbook to kf5options/qt5options

Differential Revision: https://phabricator.kde.org/D6560