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