✨(backend) add new "descendants" action to document API endpoint

We want to be able to make a search query inside a hierchical document.
It's elegant to do it as a document detail action so that we benefit
from access control.

CHANGELOG.md

@@ -34,6 +34,7 @@ and this project adheres to
 
 ## Added
 
+- ✨(backend) add new "descendants" action to document API endpoint #645
 - ✨(backend) new "tree" action on document detail endpoint #645
 - ✨(backend) allow forcing page size within limits #645
 - 💄(frontend) add error pages #643