docs: add Next.js serverExternalPackages note to server-processing page

Also removes outdated React 19 StrictMode note from Next.js page.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: YousefED

docs/content/docs/features/server-processing.mdx

@@ -52,3 +52,21 @@ const html = await editor.withReactContext(
   async () => editor.blocksToFullHTML(blocks),
 );
 ```
+
+## Next.js App Router
+
+If you're using `@blocknote/server-util` in a Next.js App Router API route (Route Handler), you need to add the BlockNote packages to `serverExternalPackages` in your `next.config.ts`:
+
+```typescript
+import type { NextConfig } from "next";
+
+const nextConfig: NextConfig = {
+  serverExternalPackages: [
+    "@blocknote/core",
+    "@blocknote/react",
+    "@blocknote/server-util",
+  ],
+};
+
+export default nextConfig;
+```

docs/content/docs/getting-started/nextjs.mdx

@@ -58,14 +58,4 @@ function App() {
 }
 ```
 
-## React 19 / Next 15 StrictMode
-
-BlockNote is not yet compatible with React 19 / Next 15 StrictMode. For now, disable StrictMode in your `next.config.ts`:
-
-```typescript
-...
-reactStrictMode: false,
-...
-```
-
 This should resolve any issues you might run into when embedding BlockNote in your Next.js React app!