Doc: Added iOS and Android information in Qt 5 introduction.

-linked to demos in iTunes and Google Play.

Task-number: QTBUG-33580
Change-Id: I6b54f6faa42bea16c29d7c6a13a4818b26f38723
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>