Doc: Move files to enable single-sourcing

Create creator-only folders for topics that are to be excluded
from the Qt Design Studio Manual.

Change-Id: I6e212c6f1f2911fd02b135fe72e6379ef95c9eee
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

doc/src/howto/creator-help.qdoc

@@ -31,9 +31,14 @@
 
 /*!
     \contentspage index.html
-    \previouspage creator-help-overview.html
     \page creator-help.html
+    \if defined(qtdesignstudio)
+    \previouspage studio-help.html
+    \nextpage studio-faq.html
+    \else
+    \previouspage creator-help-overview.html
     \nextpage creator-faq.html
+    \endif
 
     \title Using the Help Mode
 

doc/src/overview/creator-only/creator-help-overview.qdoc

@@ -32,11 +32,7 @@
 /*!
     \contentspage index.html
     \page creator-help-overview.html
-    \if defined(qtdesignstudio)
-    \previouspage creator-qml-performance-monitor.html
-    \else
     \previouspage creator-task-lists.html
-    \endif
     \nextpage creator-help.html
 
     \title Getting Help

doc/src/overview/creator-only/creator-supported-platforms.qdoc

@@ -123,5 +123,5 @@
     To build \QC from the source, see the requirements and instructions in the
     readme file that is located in the source repository.
 
-    \include overview/creator-target-platforms.qdocinc target platforms
+    \include creator-target-platforms.qdocinc target platforms
 */