One explicit boundary
The Docs Root is a theme setting, not a hidden convention. This lets a site have a landing Page, a documentation tree, and ordinary Pages without guessing which content belongs to the system.
Parent relationships are the outline
A child Page is part of the tree when it is a published descendant of the configured root. Pages outside that subtree are rendered through the normal WordPress Page template.
1selected root
npublished descendants
0custom post types required