Clarify instancing

[Jeremylinyh]

Your Godot version:
4.3
Issue description:
Novice developers (like me) may confuse scene instancing with geometry instancing, and subsequently become confused when INSTANCE_ID is 0 in shaders. I propose having a section stating the differences between scene instancing and geometry instancing, in addition to redirecting to the MeshInstance3D and GeometryInstance3d
URL to the documentation page (if already existing):
https://docs.godotengine.org/en/stable/getting_started/step_by_step/instancing.html
https://docs.godotengine.org/en/stable/tutorials/scripting/nodes_and_scene_instances.html