Doc: Restructure the QDS Manual Getting Started chapter

- Add the Use Cases topic
- Move text from other topics as necessary
- Fix the navigation links to match the new structure

Fixes: QDS-4468
Change-Id: Ic67dbe073c5be2420795c1f5049217319ab0a202
Reviewed-by: Jarko Vihriala <jarko.vihriala@qt.io>
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

doc/qtcreator/src/user-interface/creator-ui.qdoc

@@ -32,7 +32,7 @@
 /*!
     \page creator-quick-tour.html
     \if defined(qtdesignstudio)
-    \previouspage studio-installation.html
+    \previouspage {Your First UI}
     \else
     \previouspage creator-overview.html
     \endif

doc/qtcreator/src/user-interface/creator-workspaces.qdoc

@@ -26,7 +26,11 @@
 /*!
     \page creator-project-managing-workspaces.html
     \previouspage creator-open-documents-view.html
+    \if defined(qtdesignstudio)
+    \nextpage creator-project-managing-sessions.html
+    \else
     \nextpage creator-output-panes.html
+    \endif
 
     \title Managing Workspaces