Fix broken mdbook link

Add proto1_0 board

Add schematic and PCB for another prototype board, P1.0.

Main changes from the first prototype PCB (P0.0):
* Based on TI ADS1230 ADC IC rather than HX711
* More thoughtful layout of ADC
* Using Fanstel BT832 module rather than a Nordic USB dongle

This will require some software changes to get working in a future
update:
* New driver for ADS1230 and power switch
* Target nRF52832 instead of nRF52840
* Different pin layout

P0.0 protoype