Improve documentation for Skeleton3D

### Describe the project you are working on

A 3D model viewer

### Describe the problem or limitation you are having in your project

The documentation for `Skeleton3D` is very sparse. The specific problem I encountered is that the description for `localize_rests()` is 'Returns all bones in the skeleton to their rest poses', which I assumed would set the (current) global pose to the (global) rest pose, which led to some obscure bugs when trying to generate `ArrayMesh` gizmos weighted to the bones. After looking through skeleton_3d.cpp, it appears that 'global' means the skeleton's space (which is _not_ the global world space), while 'local' means the parent bone's space (I think?).

Beyond my specific problem, sparse documentation also makes it harder to manipulate weighted geometry at runtime in general, which may be relevant for animation-heavy or procedural video games.

### Describe the feature / enhancement and how it helps to overcome the problem or limitation

Add more descriptive documentation to `Skeleton3D`, perhaps describing what the difference is between the 3 `get_bone_pose()` methods is. Add documentation for related structures and methods like `Skin` and `create_skin_from_rest_transforms`, both of which are completely undocumented.

### Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Not applicable

### If this enhancement will not be used often, can it be worked around with a few lines of script?

Not applicable

### Is there a reason why this should be core and not an add-on in the asset library?

Not applicable