Doc: Add an image for a generic code sample

This image is to be used as a thumbnail in example manifest
files (used by Qt Creator) for examples that do not include
any images in their documentation.

Change-Id: I758e61579c99a5d890438e0e7dc4778d08113288
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Merge remote-tracking branch 'origin/stable' into dev

Change-Id: Ib193e4a2012fda32a2fd5aac6fe41be6eb887d2e