Doc: Add QML Todo example to the list of highlighted examples

Task-number: QTBUG-37203
Change-Id: Id9067acb70538f85c7aae02d99df77d78643309e
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>