code: def fill_content(self, page, placeholder_slot):
          
          if len(placeholder_slot) == 1:
              raise RuntimeError(placeholder_slot)
          placeholder, created = self.get_or_create_placeholder(
              page, placeholder_slot)
          self.add_plugins(page, placeholder)
docs: Add a placeholder to the page.
          Here we add a "TextPlugin" in all languages.