add workspaceoptions docbook

in systemsettings it is not possible to find the module
containing the single / double click settings using the search field
but that works in khelpcenter and on docs.kde.org
-> therefore add a workspaceoptions docbook

Differential Revision: https://phabricator.kde.org/D13022

(cherry picked from commit 6229570c3da21072fdb887e585a9b68d9bcaf11e)

backport to 5.13