Doc: Adding link to Qt 5.4 new API page.

"What's New in Qt 5.4" and the "New Classes..." pages
were recently added and edited.

Change-Id: Ieb9562bca573a6dd63d6caeb3f4462ee040ee417
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>