How do I implement a node hierarchy?
In Drupal 8 you can do this out of the box. Put a reference field in the parent and target it on the child content type. The label of the field would be "In this section". You can theme the section in a field twig of this specific field, for example field--field-reference.html.twig.

You can nest the entities, but if you reference the same content type and you leave the reference field in the view mode of the child there is a risk of an infinite loop, which will result in a time-out.