Add a sitemap page and tidy sitemap.xml: site order, x-default and build-date last-modified

next.config.ts

@@ -3,6 +3,8 @@ import type { NextConfig } from "next";
 const nextConfig: NextConfig = {
   /* config options here */
   cacheComponents: true,
+  // The sitemap's last-modified date: the day this deploy was built.
+  env: { BUILD_DATE: new Date().toISOString().slice(0, 10) },
   partialPrefetching: true,
   turbopack: {
     rules: {