Update desktopthemedetails kcm docbook to 5.6

Remove Info about Details tab and how to configure single components of a theme, feature was removed in 5.6
Add note to inform users about removed feature to avoid confusion for users, see e.g https://forum.kde.org/viewtopic.php?f=17&t=132011
Add link to techbase page with info about themedetails
Add recommended tool
REVIEW:127640