Add examples for how to integrate GLTF assets into BSN

## How can Bevy's documentation be improved?

Currently we have no examples to demonstrate how to integrate GLTF assets into BSN.

I feel a very common workflow for developers is:
- a character scene defined with BSN
- a character GLTF asset with meshes, materials, animations, etc

However, BSN and GLTF scenes aren't interoperable; it feels like we're not supposed to use GLTF scenes (WorldAssetRoot) in BSN at all, and we're supposed to use GLTF as a container for our assets.

This was a pain point for me when I migrated to BSN, and it would be nice if we had a canonical example on how we're "supposed" to integrate GLTF assets into BSN.

Start by locating the existing BSN documentation and GLTF asset examples in the Bevy repository, then compare how their workflows are currently presented. Done means the documentation includes a canonical example showing how a character scene defined with BSN uses GLTF meshes, materials, and animations.

Written by the indexing model from the issue text.