How-to: make a ALE driver

How-to that should explain the process for creating an ALE driver from no code to ISD.

Not exhaustive list of things the user should know:

- [x] Why are the drivers necessary?
- [x] What are mixins and how does ALE use them?
- [x] How do I determine what should be overwritten?
- [x] What does `Ale::load` do and why is it important during driver creation?
- [ ] What should I do if the mixin I need doesn't exist?
- [ ] direct to a separate how-to guide on mixins
- [ ] How can I test my new driver?

I imagine it would pretty long so splitting some of these to different guides might make sense.