Add bilingual manual and localized docs routes

site/decodal-site/src/pages/docs/ja/[...slug].astro

@@ -0,0 +1,20 @@
+---
+import ManualLayout from '../../../layouts/ManualLayout.astro';
+import { allDocSlugs, docTitle, renderMarkdown } from '../../../lib/docs.js';
+import '@fontsource/ibm-plex-sans-jp/400.css';
+import '@fontsource/ibm-plex-sans-jp/600.css';
+
+export function getStaticPaths() {
+  return allDocSlugs('ja').map((slug) => ({
+    params: { slug },
+    props: { slug },
+  }));
+}
+
+const { slug } = Astro.props;
+const html = renderMarkdown(slug, 'ja');
+const title = docTitle(slug, 'ja');
+---
+<ManualLayout title={`${title} — Decodalマニュアル`} active={slug} locale="ja">
+  <article class="markdown" set:html={html} />
+</ManualLayout>