Mark SSGI Lightmap attributions as example content

The attribution entries for the SSGI Lightmap example did not set
QtParts, so they fell back to the default "libs" and were reported as
if the icons and models were part of the Qt Quick 3D library itself.

Set QtParts to "examples" for all entries, and fill in the QtUsage
field where it was missing.

Pick-to: 6.11 6.12 6.12.0
Change-Id: Ib78f061b039910db5ae85c4bc38f8c083e75756a
Reviewed-by: Jonas Karlsson <jonas.karlsson@qt.io>