Doc: Define a generic thumbnail for a number of examples

Add examples that do not have any images in their documentation
into manifest-meta qdocconf file, and define a generic image to
use as a thumbnail in Qt Creator Welcome mode.

Task-number: QTBUG-31314
Change-Id: I1625bf15cbaa53e940a07b1828dc77ca5ba45ccc
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>