docs: def register(self, slug, bundle, order=1, title=None):
          """
          Registers the bundle for a certain slug.
  
          If a slug is already registered, this will raise AlreadyRegistered.
  
          :param slug: The slug to register.
          :param bundle: The bundle instance being registered.
          :param order: An integer that controls where this bundle's \
          dashboard links appear in relation to others.
          """
  
          if slug in self._registry:
              raise AlreadyRegistered('The url %s is already registered' % slug)
  
          # Instantiate the admin class to save in the registry.
          self._registry[slug] = bundle
          self._order[slug] = order
          if title:
              self._titles[slug] = title
          bundle.set_admin_site(self)