fix: serve docs demo runtime in dev

docs-site/src/pages/demo/runtime/app/index.js

@@ -0,0 +1,14 @@
+import { readFile } from "node:fs/promises";
+import path from "node:path";
+
+const runtimeAppHtml = path.join(process.cwd(), "public", "demo", "runtime", "app.html");
+
+export const prerender = true;
+
+export async function GET() {
+  return new Response(await readFile(runtimeAppHtml, "utf8"), {
+    headers: {
+      "Content-Type": "text/html; charset=utf-8",
+    },
+  });
+}