Clarify 2d/3d support

[tbillington]

It would be helpful to know if rerecast and bevy_rerecast support 2d navmeshes, and could it be used with Avian2d and landmass 2d.

The repo readme and docs pages don't explicitly mention it anywhere, and all the examples are 3d so as a reader I'm unsure what is supported.

[janhohenheim]

Oh, good call. Yeah it's all aimed at 3D, but since 2D in Bevy is just 3D with a hat on, it all works out anyways. You need to write your own Avian collider backend, but that's like... 40 lines or so? See https://github.com/janhohenheim/rerecast/blob/main/crates/avian_rerecast/src/lib.rs
Then when generating the mesh, you'll also need to set the up direction to be Vec3::Z in https://docs.rs/bevy_rerecast/latest/bevy_rerecast/struct.NavmeshSettings.html#structfield.up

So all in all, it's definitely usable in 2D and I've seen people do that successfully. But there's no official guidance on how to do that, simply because I'm a 3D dev and have little clue about 2D haha