Doc: Add a screenshot for Stack View example

Having a screenshot will make the example appear in Creator Welcome
mode as well.

Change-Id: I6ac810d4baeb6e3866138baaf2eb727168055b72
Reviewed-by: Caroline Chao <caroline.chao@digia.com>

src/controls/doc/src/qtquickcontrols-examples.qdoc

@@ -86,6 +86,7 @@
     \title Qt Quick Controls - Stack View Example
     \ingroup qtquickcontrols_examples
     \brief An example for a stack-based navigation model.
+    \image qtquickcontrols-example-stackview.png
 
     This example shows how to implement a stack-based navigation model
     in a Qt Quick application using a \l{StackView} component from