Doc: Re-add external link to Qt Quick 2D Renderer

Even though the functionality was integrated into Qt Quick, it's still
being linked to from "What's New" pages for earlier versions.

Change-Id: I98c16333bef81b946d8f4bcf9667383aee383b11
Reviewed-by: Venugopal Shivashankar <Venugopal.Shivashankar@qt.io>