On our platform, everything is a card (rather than a document), and if you want an actual structured hierarchy, you can nest cards inside other cards. Each card can have multiple parents, so if you are collaborating with others you can easily have the same "card" in different places to keep yourself organized. In that way it works similar to symbolic links in a filesystem, except it's nesting so there are no folders – just cards all the way down.
At the same time, because your cards are not actually siloed into different folders, we have things like the "home", which has all the cards you've ever made so that you can easily filter your entire library by tag / author / etc. and see all of those cards at once.
Unlike the OC solution, however, Supernotes is not self-hosted, and so probably isn't as appealing to a lot of the techie HN crowd.