Move hellogles3 docs to where the rest of these files live

And expand the note in the openglwindow example to point to
hellogles3 as well.

Change-Id: I2c0ee9b83fa59752d937c57eaf0194f0d0a8a7d9
Reviewed-by: Christian Strømme <christian.stromme@qt.io>
(cherry picked from commit 7c23aa6bc5e5d7b6e891fb0c30a72091a60c79eb)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

examples/opengl/doc/src/openglwindow.qdoc

@@ -12,6 +12,8 @@
 
     \note This is a low level example of how to use QWindow with OpenGL.
     In practice you should consider using the higher level QOpenGLWindow class.
+    See the \l{Hello GLES3 Example} for a demonstration of the QOpenGLWindow
+    convenience class.
 
     \section1 OpenGLWindow Super Class