document firmware timing stuff

[pompydev]

• measure exact timing by switching a pin from high to low and measuring with oscilloscope since defmt might add delay
• signal propagation chain sensor-mux-adc
• time required for each refresh rate
  • 8000 Hz: 125 us
  • 4000 Hz: 250 us
  • 2000 Hz: 500 us
  • 1000 Hz: 1000 us (=1 ms)
• mux switch duration
• ADC scan time
• data processing time
• USB communication
• margin