Doc: Add generic code sample image to qhp

Changes from fff32616 to qtdoc-qhp.qdocconf were lost during
merge as that file was deleted in the target branch.

Add the changes to qtdoc.qdocconf instead.

Change-Id: If59670d43ecab31ba1912f4ffe50266417374714
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>