Add docbook content which was lost during a merge and more cleanup

Summary:
45c37a64 lost some docbook changes from 7034b50c, 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