Doc: Add skeleton for Qt 3D Materials example docs

Without this fix, Qt Creator tags these examples "broken"
and hides them in the Examples tab in the Welcome mode.
More documentation should be added later.

Change-Id: Iddfb78a8df30e38482e89e7a9ac87a5ef83966a3
Reviewed-by: Paul Lemire <paul.lemire@kdab.com>

examples/qt3d/materials-cpp/doc/src/materials-cpp.qdoc

@@ -29,4 +29,11 @@
     \example materials-cpp
     \title Qt 3D: Materials C++ Example
     \ingroup qt3d-examples-cpp
+    \brief A C++ application that demonstrates using materials.
+
+    \image materials-cpp.png
+
+    \e {Materials} demonstrates using the Qt 3D material system.
+
+    \include examples-run.qdocinc
 */

examples/qt3d/materials/doc/src/materials.qdoc

@@ -29,4 +29,11 @@
     \example materials
     \title Qt 3D: Materials QML Example
     \ingroup qt3d-examples-qml
+    \brief A QML application that demonstrates using materials.
+
+    \image materials.png
+
+    \e {Materials} demonstrates using the Qt 3D material system.
+
+    \include examples-run.qdocinc
 */