Document new buttons and LEDs #137

Problem Description

The new prototype has several new buttons and LEDs. Some are self-explanatory (RESET), while others could use some documentation explaining their purpose.

In particular:

• The BOOT button. Through trial-and-error I found out that keeping BOOT pressed while resetting the device makes it boot into bootloader mode.
• What does the POWER button do?
• What is the purpose of the four white LEDs mentioned in White LEDs next to battery connector too bright #135 ?