Document how to work with Avian Sensors and CollisionLayers

[janhohenheim]

Follow-up to #213
This was requested on Discord.

While Sensors don't have much of a Blenvy-specific quirk, CollisionLayers require some boilerplate because of a lack of bitflag support. This comes up quickly when trying to work with sensors, as you usually only want them to collide with specific layers (notably NOT the default "layer", which is just enabling all collisions). Hence why these two should be taught in tandem.

My personal preference is creating an enum called CollisionLayerPreset that is added on the Blender side and then inserts the actual CollisionLayers in an on_add hook. More sophisticated versions can even validate whether the entity they were added to also contains a Collider or ColliderConstructor.