Awesome, I've been reading through these.
I noticed you jumped directly to "here's an 8x darlington package," but maybe a separate page beforehand would be a good place to introduce the general topic of using two different power supplies.
A quick schematic to show what a darlington pair is, and does.
The use of a step-up, either by darlington, or a hexfet (or an example for both), to allow a VDD Arduino drive a big load on 12V or more. Including a reversed diode if the load is inductive.
The use of step-down to allow a 5V Arduino control a 3.3V device.
And so on.