Update documentation for kcontrol desktop theme (now plasma theme)
Summary: Some part were copied from kcontrol icons. Need proofreading
Test Plan: checkxml index.docbook and meinproc5 index.docbook
Reviewers: Documentation, yurchor
Reviewed By: Documentation, yurchor
Subscribers: yurchor, plasma-devel, kde-doc-english
Tags: Plasma, Documentation
Differential Revision: https://phabricator.kde.org/D19618