Add an example on placing decals during gameplay in Using decals

[Calinou]

Your Godot version: 4.0

Issue description: In #6178, I originally had an example of placing decals during gameplay (according to the mouse cursor position). I removed it as it was incomplete, but we should try to get a complete version back in. This version should take the surface's collision normal into account for orienting the decal, rather than the camera rotation.

URL to the documentation page: https://docs.godotengine.org/en/latest/tutorials/3d/using_decals.html