✨(backend) allow the duplication of subpages

Adds a new with_descendants parameter to the doc duplication API.
The logic of the duplicate() method has been moved to a new
internal _duplicate_document() method to allow for recursion.
Adds unit tests for the new feature.

src/backend/core/api/serializers.py

@@ -591,10 +591,13 @@ class LinkDocumentSerializer(serializers.ModelSerializer):
 class DocumentDuplicationSerializer(serializers.Serializer):
     """
     Serializer for duplicating a document.
-    Allows specifying whether to keep access permissions.
+    Allows specifying whether to keep access permissions,
+    and whether to duplicate descendant documents as well
+    (deep copy) or not (shallow copy).
     """
 
     with_accesses = serializers.BooleanField(default=False)
+    with_descendants = serializers.BooleanField(default=False)
 
     def create(self, validated_data):
         """