Update the Repeater3D docs

Check the documentation in the Repeater3D topic for any needed updates (https://doc-snapshots.qt.io/qtdesignstudio/studio-3d-repeater-3d.html). Update the pictures.

The example described in the "Adding a Repeater3D component with a List Model" section doesn't seem to work - nothing is drawn to the 3D/2D views.