[Docs] Update the system overview image #4
Description
We should use the UART pins:
- GPIO 0 - TX (power-system) -> GPIO 1 - RX (onboard-computer)
- GPIO 1 - RX (power-system) -> GPIO 0 - TX (onboard-computer)
We're also adding comments for GPIO 20 & 21 and maybe we should add docs for using it for debugging too.