The documents are useless are they are generic about the chips and there is no schematic for this specific board model. Anyways if anybody needs, the only way is via voltage divider to ADC pin (GPIO32 to 39) using two 100K Ohm resistors... the voltage can be calculated using the formula: (readAnalog(pin) / 4095) * 3.3 * 2 (also adding a fixed value to calibrate the result is suggested, based on measurements with multimeter)