add nextjs app router example

docs/receiving/verifying-payloads/how.mdx

@@ -467,6 +467,42 @@ export default async function handler(req, res) {
 }
 ```
 
+### Node.js (Next.js 13 App Router)
+
+```js
+import { Webhook } from "svix";
+
+const webhookSecret: string = process.env.WEBHOOK_SECRET || "your-secret";
+
+export async function POST(req: Request) {
+  const svix_id = req.headers.get("svix-id") ?? "";
+  const svix_timestamp = req.headers.get("svix-timestamp") ?? "";
+  const svix_signature = req.headers.get("svix-signature") ?? "";
+
+  const body = await req.text();
+
+  const sivx = new Webhook(webhookSecret);
+
+  let msg;
+
+  try {
+    msg = sivx.verify(body, {
+      "svix-id": svix_id,
+      "svix-timestamp": svix_timestamp,
+      "svix-signature": svix_signature,
+    });
+  } catch (err) {
+    return new Response("Bad Request", { status: 400 });
+  }
+
+  console.log(msg);
+
+  // Rest
+
+  return new Response("OK", { status: 200 });
+}
+```
+
 ### Node.js (Netlify Functions)
 
 ```js