Release the Skin node

[stefaniapedrazzi]

The Skin node is still experimental and it would be good to make it public and document it.
From the latest feedback I received, using FBX format for the human model (mesh and rigged skeleton) and BVH motion fine is the good way to go.

Previous work on documentation:
cyberbotics/webots-doc#96
Remaining issues:
https://github.com/cyberbotics/webots-dev/issues/4063

[BenjaminDeleze]

I think it was fixed in #3566