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