Update advanced-config.rst

I think the documentation is missing the manifest definition step

Author: iampedropiedade

frontend/encore/advanced-config.rst

@@ -118,6 +118,21 @@ Next, define the output directories of each build:
             firstConfig: '%kernel.project_dir%/public/first_build'
             secondConfig: '%kernel.project_dir%/public/second_build'
 
+
+Also define the asset manifests for each build:
+
+.. code-block:: yaml
+
+    # config/packages/assets.yaml
+    framework:
+        assets:
+            packages:
+                first_build:
+                    json_manifest_path: '%kernel.project_dir%/public/first_build/manifest.json'
+                second_build:
+                    json_manifest_path: '%kernel.project_dir%/public/second_build/manifest.json'
+
+
 Finally, use the third optional parameter of the ``encore_entry_*_tags()``
 functions to specify which build to use: