Doc: Add What's New in Qt 5.8 information

Content based on https://wiki.qt.io/New_Features_in_Qt_5.8



Change-Id: I021b7853e532dfad547e5dc7b90815f2d17da87b
Reviewed-by: Venugopal Shivashankar <Venugopal.Shivashankar@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>