feat(sitemap): add support for "lastmod" (#9954)

packages/docusaurus-utils/src/routeUtils.ts

@@ -0,0 +1,19 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import type {RouteConfig} from '@docusaurus/types';
+
+/**
+ * Recursively flatten routes and only return the "leaf routes"
+ * Parent routes are filtered out
+ */
+export function flattenRoutes(routeConfig: RouteConfig[]): RouteConfig[] {
+  function flatten(route: RouteConfig): RouteConfig[] {
+    return route.routes ? route.routes.flatMap(flatten) : [route];
+  }
+  return routeConfig.flatMap(flatten);
+}