docs: set-up sapper template

docs/src/routes/blog/[slug].json.js

@@ -0,0 +1,21 @@
+import posts from "./_posts.js";
+
+const lookup = new Map();
+
+posts.forEach((post) => {
+  lookup.set(post.slug, JSON.stringify(post));
+});
+
+export function get(req, res, next) {
+  // the `slug` parameter is available because
+  // this file is called [slug].json.js
+  const { slug } = req.params;
+
+  if (lookup.has(slug)) {
+    res.writeHead(200, { "Content-Type": "application/json" });
+    res.end(lookup.get(slug));
+  } else {
+    res.writeHead(404, { "Content-Type": "application/json" });
+    res.end(JSON.stringify({ message: `Not found` }));
+  }
+}