✨(backend) add "tree" action on document API endpoint

We want to display the tree structure to which a document belongs
on the left side panel of its detail view. For this, we need an
endpoint to retrieve the list view of the document's ancestors
opened.

By opened, we mean that when display the document, we also need to
display its siblings. When displaying the parent of the current
document, we also need to display the siblings of the parent...

CHANGELOG.md

@@ -32,7 +32,8 @@ and this project adheres to
 
 ## Added
 
-- ✨(backend) allow forcing page size within limits
+- ✨(backend) new "tree" action on document detail endpoint #645
+- ✨(backend) allow forcing page size within limits #645
 - 💄(frontend) add error pages #643
 - 🔒️ Manage unsafe attachments #663
 - ✨(frontend) Custom block quote with export #646