I've prototyped this mentally too. If I were going to do it, I think I'd write myself a modification to a Wiki engine that can detect when links only go to simpler things, and highlight those. (You don't want to "ban" links going up to harder things, but you may want to isolate them in a dedicated section or something.)

You might be able to bootstrap off of wikipedia content.

The thought that provoked this in my own head was the observation that Wikipedia is virtually useless for learning anything mathematical because so many of the links on any given article head "up", rather than "down", and a casual user can't figure out which is which.