Doc: Describe adding the import path

Also add the import path to the example main.cpp file.

Task-number: QDS-805
Change-Id: I990e5dc364d610a4cef5697b3355147f9e5fcde1
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

doc/examples/progressbar/main.cpp

@@ -9,6 +9,7 @@ int main(int argc, char *argv[])
     QGuiApplication app(argc, argv);
 
     QQuickView view;
+    view.engine->addImportPath("qrc:/qml/imports");
     view.setSource(QUrl("qrc:/qml/ProgressBar.ui.qml"));
     if (!view.errors().isEmpty())
         return -1;