Update keys docbook to 5.12

proofread + update
bump date + releseinfo
add a para about adding components from https://phabricator.kde.org/D2111

Differential Revision: https://phabricator.kde.org/D11690

(cherry picked from commit 4edcfeb171f3d32fdbb869855112ee7913ab00c2)

backport to 5.12 LTS