📈(backend) capture an event when a document is left

When a user leave a document, we want to capture an event.

src/backend/core/api/viewsets.py

@@ -2602,6 +2602,8 @@ class DocumentViewSet(
             )
             raise
 
+        posthog_capture(PosthogEventName.DOC_LEFT, request.user, {}, document=document)
+
         return drf.response.Response(status=drf.status.HTTP_204_NO_CONTENT)