add admin page event doc

docs/tree/page.md

@@ -116,3 +116,35 @@ public function getMyBlock(): Block
     return $this->getBlock('myBlock', CollectionBlock::class);
 }
 ```
+
+## Event
+
+When a page is getting edited, options can be set this way:
+
+
+```
+// src/EventSuscriber/PageEventSubscriber.php
+namespace App\EventSuscriber;
+
+use App\Core\Event\Page\PageEditEvent;
+use App\Entity\Page\YourPage;
+
+class PageEventSubscriber implements EventSubscriberInterface
+{
+    public static function getSubscribedEvents()
+    {
+        return [
+            PageEditEvent::FORM_INIT_EVENT => ['onFormInit'],
+        ];
+    }
+
+    public function onFormInit(PageEditEvent $event)
+    {
+        if ($event->getPage() instanceof YourPage) {
+            $event->addPageBuilderOptions([
+                // options
+            ]);
+        }
+    }
+}
+```