Draft of Vegimeter 2 Rev 3 Board with docs.

This version supports a Parallax QuickStart board directly attached to
the greatly simplified main Vegimeter board. Additionally, we have a
much greater heater power capacity thanks to 3x relays and independent
and parallel voltage rails. After studying the prior revision for a
week, I moved the parts around to optimize heat flow and reduce the
trace length of higher power traces. I made all higher power traces
(and a few others) very wide to improve power efficiency and safety.
Cleanup of vegimeter 2 rev 3