Add docbook content which was lost during a merge and more cleanup
Summary:
45c37a64e1cc lost some docbook changes from 7034b50c5c9e, let's restore
them.
Also bump date and releaseinfo, and remove entities which now come from
kdoctools.
Thanks to @gregormi for the initial version of the patch, and @lueck for
help with the docbook.
Depends on D10529
Test Plan:
checkXML5 index.docbook
Proofread in KHelpCenter.
Reviewers: Spectacle, gregormi, Documentation, lueck
Reviewed By: Documentation, lueck
Subscribers: kde-doc-english, lueck, ngraham, rkflx
Differential Revision: https://phabricator.kde.org/D10542