code: def add_404_page(app):
      
      is_epub = isinstance(app.builder, EpubBuilder)
      config_pages = app.config.html_additional_pages
  
      if not is_epub and "404" not in config_pages:
          yield ("404", {}, "404.html")
docs: Build an extra ``404.html`` page if no ``"404"`` key is in the
      ``html_additional_pages`` config.