Doc: Update global illumination overview

-Mention SSGI as an option for global illumination
-Link to SSGI example
-Removed note about lightmaps being in tech preview in 6.4

Task-number: QTBUG-145979
Pick-to: 6.12
Change-Id: I6fa56123d0890e72a6abff00c01e4adb1bdd63f7
Reviewed-by: Jonas Karlsson <jonas.karlsson@qt.io>
Reviewed-by: Andy Nichols <andy.nichols@qt.io>
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>