docs(rolldown): add manualChunks to advancedChunks migration guide (#20161)

Author: cexbrayat

docs/guide/rolldown.md

@@ -99,6 +99,42 @@ Rolldown throws an error when unknown or invalid options are passed. Because som
 
 If you don't pass the option in yourself, this must be fixed by the utilized framework. You can suppress this error in the meantime by setting the `ROLLDOWN_OPTIONS_VALIDATION=loose` environment variable.
 
+### API Differences
+
+#### `manualChunks` to `advancedChunks`
+
+Rolldown does not support the `manualChunks` option that was available in Rollup. Instead, it offers a more fine-grained setting via the [`advancedChunks` option](https://rolldown.rs/guide/in-depth/advanced-chunks#advanced-chunks), which is more similar to webpack's `splitChunk`:
+
+```js
+// Old configuration (Rollup)
+export default {
+  build: {
+    rollupOptions: {
+      output: {
+        manualChunks(id) {
+          if (/\/react(?:-dom)?/.test(id)) {
+            return 'vendor'
+          }
+        }
+      }
+    }
+  }
+}
+
+// New configuration (Rolldown)
+export default {
+  build: {
+    rollupOptions: {
+      output: {
+        advancedChunks: {
+          groups: [{ name: 'vendor', test: /\/react(?:-dom)?// }]
+        }
+      }
+    }
+  }
+}
+```
+
 ## Performance
 
 `rolldown-vite` is focused on ensuring compatibility with the existing ecosystem, so defaults are geared towards a smooth transition. You can get further performance gains by switching over to faster Rust-based internal plugins and other customizations.