Update kfontview docbook to 5.12

bump date + releaseinfo
remove obsolete entities kfontview, kappname, package - either already defined in kdoctools or unused in kf5
fix name of systemsettings module and add info about the extended managemanet capabilities

Differential Revision: https://phabricator.kde.org/D11832

(cherry picked from commit 1133795f6995cf7f67ccc3a9bf8d39d40eabde55)

backport to stable 5.12 lts