🚸(backend) add onboarding docs for new users

Adds two methods to allow new users to start with some docs.

User._handle_onboarding_documents_access() gives READER access to
each document listed in settings.USER_ONBOARDING_DOCUMENTS.

User._duplicate_onboarding_sandbox_document() creates a local copy
of the sandbox document specified in
settings.USER_ONBOARDING_SANDBOX_DOCUMENT.

CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to
 - ✨(backend) manage reconciliation requests for user accounts #1878
 - 👷(CI) add GHCR workflow for forked repo testing #1851
 - ✨(backend) allow the duplication of subpages #1893
+- ✨(backend) Onboarding docs for new users #1891
 
 ### Changed