Doc: Remove Layouts from Qt Quick Controls index page

Qt Quick Layouts is a module by itself and it is misleading to
list its items here.

Add a note instead to say that Qt Quick Controls can be used with
Qt Quick Layouts.

Change-Id: I0e0a506f5ff7f2702126388fa60ff075ee2cbb4e
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>