Adopt Sangaboard v0.5 as default electronics

The OpenFlexure Microscope instructions are currently a bit vague about what electronics should be used. This was initially intentional, but can be quite unhelpful. This OFEP proposes we adopt Sangaboard v5 as the default, with discrete boards (Arduino Nano or Pi Pico plus Darlington Pair drivers and a 5mm LED) as a supported alternative, when custom PCBs are unavailable.