docs: def register(cls):
      """Register an :class:`Entity` as a commentable class.
  
      Can be used as a class decorator:
  
      .. code-block:: python
  
        @comment.register
        class MyContent(Entity):
            ...
      """
      if not issubclass(cls, Entity):
          raise ValueError("Class must be a subclass of abilian.core.entities.Entity")
  
      Commentable.register(cls)
      return cls