Doc: Updated the intro page

Added high-level intro text about the important features in Qt 5.x
so that the reader will get a glipmse of these features in the intro
page.

Task-number: QTBUG-33596
Change-Id: I286e6a32d1cf5e4a7e3c5f7c08e6757d7b3b592c
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>