Update categories docbook (KMM)

Summary:
Update Categories chapter of Handbook for 5.x

Merge branch '5.0' into docs

(Revision initially created including irrelevant commits. Hopefully it now just has the docbook updates.)

Test Plan: Ran CheckXML5 and meinproc5

Reviewers: kde-doc-english, KMyMoney, yurchor

Reviewed By: yurchor

Subscribers: yurchor, KMyMoney, kde-doc-english

Tags: Documentation

Differential Revision: https://phabricator.kde.org/D29428