[Docs] Collapse menu items (#302)

I've just copied the recent change on Sylius documentation side, cause
it's not documented here too.

On Sylius documentation
<img width="840" height="275" alt="image"
src="https://github.com/user-attachments/assets/9fef7106-96f5-44ef-851d-b75d4e2a6a39"
/>

On Sylius stack documentation preview
<img width="840" height="275" alt="image"
src="https://github.com/user-attachments/assets/539bc3c8-c8eb-4ba4-bc2e-5bcfa50f1ad2"
/>

Author: loic425

docs/cookbook/admin_panel/menu.md

@@ -88,3 +88,19 @@ final readonly class MenuBuilder implements MenuBuilderInterface
     }
 }
 ```
+
+{% hint style="success" %}
+**🧠 Collapse your custom menu by default**
+
+It's possible to expand your parent menu category on page load by default. For that, you have to set the `setExtra` attribute like this:
+
+```php
+$library = $menu
+    ->addChild('library')
+    ->setLabel('app.ui.library')
+    ->setLabelAttribute('icon', 'tabler:books')
+    ->setExtra('always_open', true);
+```
+
+However, ensure that you set the attribute in the parent menu, not in one of the child menu items.
+{% endhint %}