Add SEO and agent-readable documentation

site/decodal-site/astro.config.mjs

@@ -1,7 +1,47 @@
 import { defineConfig } from 'astro/config';
+import sitemap from '@astrojs/sitemap';
+
+const site = 'https://decodal.hareworks.net';
+
+function alternateLinks(url) {
+  const pathname = new URL(url).pathname;
+  if (pathname === '/' || pathname === '/ja/') {
+    return [
+      { url: `${site}/`, lang: 'en' },
+      { url: `${site}/ja/`, lang: 'ja' },
+      { url: `${site}/`, lang: 'x-default' },
+    ];
+  }
+
+  const match = pathname.match(/^\/docs\/(en|ja)\/(.*)$/);
+  if (!match) return undefined;
+  const localizedPath = match[2];
+  return [
+    { url: `${site}/docs/en/${localizedPath}`, lang: 'en' },
+    { url: `${site}/docs/ja/${localizedPath}`, lang: 'ja' },
+    { url: `${site}/docs/en/${localizedPath}`, lang: 'x-default' },
+  ];
+}
 
 export default defineConfig({
+  site,
   output: 'static',
+  integrations: [
+    sitemap({
+      filter: (page) => {
+        const pathname = new URL(page).pathname;
+        return pathname !== '/docs/'
+          && pathname !== '/404.html'
+          && pathname !== '/404/'
+          && !pathname.endsWith('.md')
+          && !pathname.endsWith('.txt');
+      },
+      serialize(item) {
+        const links = alternateLinks(item.url);
+        return links ? { ...item, links } : item;
+      },
+    }),
+  ],
   build: {
     assets: 'assets',
   },