Diagram Ember Learn site architecture

This diagram should probably go somewhere else too, but we need to figure out where. In the meantime, it can go in the README of this repo.

Anker is working on turning these sketches into a diagram. The final result should be an SVG so anyone can edit it regardless of their software.

Not included in this diagram, but should be:
- WIP https://emberjs.github.io/rfcs/

cc @EndangeredMassa this might eventually be helpful for your help-wanted/contributors app.