Document power draw of the official case fan and the active cooling fan

[C0rn3j]

As per raspberrypi/rpi-eeprom#545 (comment), power draw is shared between USB and the fan header, making knowing the power draw of the fans important.

The documentation is missing here for the fan case and here for the active cooler

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[C0rn3j]

@aallan @LizUpton @nathan-contino

Would be nice if the stale bot marked thing stale instead of stale+spam+autoclose.

[nathan-contino]

Not so fast, stalebot. We'll get to this shortly. Sorry for the delay.