Doc: Add What's new page for 5.11

Based on the wiki entries here:
https://wiki.qt.io/New_Features_in_Qt_5.11



Also added links to the new page from relevant pages, based on the
similar commit for version 5.9.

Task-number: QTBUG-67560
Change-Id: I927db8fec99fd9949534e54f0e3378af77f58a53
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>