docs: def init_app(self, app):
          """Flask application initialization.
  
          :param app: The Flask application.
          :returns: The :class:`invenio_pages.ext.InvenioPages` instance
              initialized.
          """
          self.init_config(app)
  
          self.wrap_errorhandler(app)
          app.extensions['invenio-pages'] = _InvenioPagesState(app)
  
          return app.extensions['invenio-pages']