Doc: Describe converting Qt Quick UI Prototypes to Applications

Qt Quick UI Prototype projects (.qmlproject) are used in Qt Design
Studio, but they must be converted to Qt Quick Application projects
for building and running them in Qt Creator.

Add example files and quote code from them.

Task-number: QDS-357
Change-Id: Ib0bf1b342ec54faeca108e5586389730a85b5116
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

doc/examples/progressbar/qml/ProgressBar.qmlproject

@@ -0,0 +1,20 @@
+/* File generated by Qt Creator */
+
+import QmlProject 1.1
+
+Project {
+    mainFile: "ProgressBar.ui.qml"
+
+    /* Include .qml, .js, and image files from current directory and subdirectories */
+    QmlFiles {
+        directory: "."
+    }
+    JavaScriptFiles {
+        directory: "."
+    }
+    ImageFiles {
+        directory: "."
+    }
+    /* List of plugin directories passed to QML runtime */
+    // importPaths: [ "../exampleplugin" ]
+}