Setup chapter skips `keys`

[aitorres]

On the Setup chapter, there should be a section mentioning how keys / the keypad is represented on this implementation.

Up to the Stack section, keys are not mentioned. Then, the section that comes right after which is the Timers already includes the keys as part of the Emu struct with no explanation. I actually missed it until my code editor complained of missing keys afterwards in the process.

[aquova]

Thanks for pointing this out, I think I must have had this section at some point, but accidentally removed it. I've rewritten a section setting that up.

Fixed in 2ed1540