Fix v8 migration guide for Next.js to align with wizard (#12195)

Author: Luca Forstner

platform-includes/migration/javascript-v8/important-changes/javascript.nextjs.mdx

@@ -12,48 +12,39 @@ If you need to support older versions of Node.js or Next.js, please use Sentry N
 
 ### Updated SDK initialization
 
-With `8.x` the Next.js SDK will no longer support the use of `sentry.server.config.js|ts` and `sentry.edge.config.js|ts` files. Instead, please initialize the Sentry Next.js SDK for the server-side in a Next.js [instrumentation hook](https://nextjs.org/docs/app/api-reference/file-conventions/instrumentation). Usage of `sentry.client.config.ts|js` is still supported and encouraged for initializing the client-side SDK.
+With `8.x` the Next.js SDK will require an additional `instrumentation.ts` file to execute the `sentry.server.config.js|ts` and `sentry.edge.config.js|ts` modules to initialize the SDK for the server-side.
+The `instrumentation.ts` file is a Next.js native API called [instrumentation hook](https://nextjs.org/docs/app/api-reference/file-conventions/instrumentation).
 
-To start using the Next.js built-in hooks, follow these steps:
+To start using the Next.js instrumentation hook, follow these steps:
 
-1. First, enable the Next.js instrumentation hook by setting the [`experimental.instrumentationHook`](https://nextjs.org/docs/app/api-reference/next-config-js/instrumentationHook) to true in your `next.config.js`.
+1. First, enable the Next.js instrumentation hook by setting the [`experimental.instrumentationHook`](https://nextjs.org/docs/app/api-reference/next-config-js/instrumentationHook) to true in your `next.config.js`. (This step is no longer required with Next.js 15)
 
 ```JavaScript {filename:next.config.js} {2-4}
 module.exports = {
   experimental: {
-    instrumentationHook: true,
+    instrumentationHook: true, // Not required on Next.js 15+
   },
 }
 ```
 
 2. Next, create a `instrumentation.ts|js` file in the root directory of your project (or in the src folder if you have have one).
 
-3. Now, export a register function from the `instrumentation.ts|js` file and call `Sentry.init()` inside of it:
+3. Now, export a register function from the `instrumentation.ts|js` file and import your `sentry.server.config.js|ts` and `sentry.edge.config.js|ts` modules:
 
 ```JavaScript {filename:instrumentation.js}
 import * as Sentry from '@sentry/nextjs';
 
-export function register() {
+export async function register() {
   if (process.env.NEXT_RUNTIME === 'nodejs') {
-    // this is your Sentry.init call from `sentry.server.config.js|ts`
-    Sentry.init({
-      dsn: '___PUBLIC_DSN___',
-      // Your Node.js Sentry configuration...
-    });
+    await import('./sentry.server.config');
   }
 
-  // This is your Sentry.init call from `sentry.edge.config.js|ts`
   if (process.env.NEXT_RUNTIME === 'edge') {
-    Sentry.init({
-      dsn: '___PUBLIC_DSN___',
-      // Your Edge Runtime Sentry configuration...
-    });
+    await import('./sentry.edge.config');
   }
 }
 ```
 
-4. Last, you can delete the `sentry.server.config.js|ts` and `sentry.edge.config.js|ts` files. Please remember to keep your `sentry.client.config.ts|js` file for client-side SDK initialization.
-
 <Alert level="warning">
 
 If you are using a [Next.js custom server](https://nextjs.org/docs/pages/building-your-application/configuring/custom-server), the `instrumentation.ts|js` hook is not called by Next.js so SDK instrumentation will not work as expected. See the [troubleshooting section](#nextjs-custom-server) for more information.