How do I implement a node hierarchy? This may be a trivial question, but I haven't been able to find a clear answer how to do this in Drupal 8. 

How can I organize nodes in a hierarchy like this? 

Node
- child
- child
- - child
- child
Node
- child


I know about Node Hierarchy and several other modules which seem to offer this functionality, but I haven't found anything for D8. 

It would also be good if this would work in a way allowing for listing chilren of a given node (like "In this section:", followed by a listing of child nodes.

Still, this seems like such a must-have ability to me, it's possible that Drupal can do it out of the box and I just don't know about it. Am I missing something?
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.