Expose MeshInstance3D.get_skin_reference and add docs

[lyuma]

SkinReference was missing documentation.

Also, there was not an obvious way to access the skin_ref used internally by a particular MeshInstnace3D node, so this adds a getter so the actual SkinReference object used by the MeshInstance3D can be accessed without going through Skeleton.

There was a discussion on the #animation channel on RocketChat. CC @smix8

[smix8]

Yes for the exposed function.

Without this the RenderingServer skeleton API can not be used in scripting with the default skin because there is no access to it.

[akien-mga]

@lyuma Ping for documentation style fixes before merging.

Edit: Amended myself.

[akien-mga]

Thanks!