docs: def jinja_env(self):
          """Create a sandboxed Jinja environment."""
          if self._jinja_env is None:
              self._jinja_env = SandboxedEnvironment(
                  extensions=[
                      'jinja2.ext.autoescape', 'jinja2.ext.with_', ],
                  autoescape=True,
              )
              self._jinja_env.globals['url_for'] = url_for
              # Load whitelisted configuration variables.
              for var in self.app.config['PAGES_WHITELIST_CONFIG_KEYS']:
                  self._jinja_env.globals[var] = self.app.config.get(var)
          return self._jinja_env