✨(api) add API route to fetch document content

This allows API users to process document content, enabling the
use of Docs as a headless CMS for instance, or any kind of document
processing. Fixes #1206.

src/backend/core/models.py

@@ -783,6 +783,7 @@ class Document(MP_Node, BaseModel):
             "children_list": can_get,
             "children_create": can_create_children,
             "collaboration_auth": can_get,
+            "content": can_get,
             "cors_proxy": can_get,
             "descendants": can_get,
             "destroy": can_destroy,