Doc: Add links from Qt Quick tutorials to UI design overview

...in Qt docs. Because a user felt the information is missing.

Change-Id: I76a7820974073651e6632b0f8e705655377b92c0
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

doc/src/qtquick/creator-mobile-app-tutorial.qdoc

@@ -45,6 +45,8 @@
 
     \image creator_android_tutorial_ex_app.png
 
+    For more information about the UI choices you have, see \l{User Interfaces}.
+
     \section1 Setting up the Development Environment
 
     To be able to build and run the application on a mobile device, you must

doc/src/qtquick/qtquick-uiforms-tutorial.qdoc

@@ -36,6 +36,7 @@
     to implement a simplified version of the \l{Qt Quick Controls - UI Forms}
     {UI Forms example}, which provides an interface to a customer database and
     is purely written in QML and JavaScript.
+    For more information about the UI choices you have, see \l{User Interfaces}.
 
     \image qmldesigner-uiforms-example.png