Add the two diagrams I always draw during training

[danielpcox]

(Also consider replacing the bubble chart that explains how configs link together with a similar one that shoes the directionality of the links between config files.)

Here are snapshots of the diagrams I always draw, from the first training class.

They have improved a bit since then (one thing I notice now is that I usually label the request matching points with the corresponding related config file, in the second diagram), and during training they're living pictures that change a lot to aid in explanation.

Anyway, since they're a recurring ad hoc deal, they should probably be cleaned up and put into the text itself, if only because lots of people go through the training material without actually attending the class, and they're missing out on some clarification.