✨(backend) create a dedicated endpoint to update document content

We want a dedicated endpoint to update a document content. Previously,
updating the content was made on the update action shared with all other
document's properties. When the title is updated, the response contains
the content, so a call to the s3 storage is made and we don't want this.
Isolating the content update will allow us in the next commit to remove
the content from the Document serializer.

src/backend/core/models.py

@@ -1309,6 +1309,7 @@ class Document(MP_Node, BaseModel):
             "collaboration_auth": can_get,
             "comment": can_comment,
             "formatted_content": can_get,
+            "content_patch": can_update,
             "cors_proxy": can_get,
             "descendants": can_get,
             "destroy": can_destroy,