✨(backend) add django-treebeard to allow tree structure on documents

We choose to use Django-treebeard for its quality, performance and
stability. Adding tree structure to documents is as simple as
inheriting from the MP_Node class.

src/backend/core/api/serializers.py

@@ -287,7 +287,7 @@ class ServerCreateDocumentSerializer(serializers.Serializer):
                 {"content": ["Could not convert content"]}
             ) from err
 
-        document = models.Document.objects.create(
+        document = models.Document.add_root(
             title=validated_data["title"],
             content=document_content,
             creator=user,

src/backend/core/api/viewsets.py

@@ -431,7 +431,11 @@ class DocumentViewSet(
 
     def perform_create(self, serializer):
         """Set the current user as creator and owner of the newly created object."""
-        obj = serializer.save(creator=self.request.user)
+        obj = models.Document.add_root(
+            creator=self.request.user,
+            **serializer.validated_data,
+        )
+        serializer.instance = obj
         models.DocumentAccess.objects.create(
             document=obj,
             user=self.request.user,