✨(backend) allow the duplication of subpages

Adds a new with_descendants parameter to the doc duplication API.
The logic of the duplicate() method has been moved to a new
internal _duplicate_document() method to allow for recursion.
Adds unit tests for the new feature.

CHANGELOG.md

@@ -14,6 +14,7 @@ and this project adheres to
 - ✨(frontend) Can print a doc #1832
 - ✨(backend) manage reconciliation requests for user accounts #1878
 - 👷(CI) add GHCR workflow for forked repo testing #1851
+- ✨(backend) allow the duplication of subpages #1893
 
 ### Changed