Add expansion board manual

Add a manual for the expansion board.
https://github.com/pycom/pycom-documentation/blob/master/chapter/datasheets/boards/expansion.md

A good start would be the previous expansion boards manual: https://github.com/wipy/wipy/blob/master/docs/User_manual_exp_board.pdf

[Bucknalla]

Hi @r-uhlig what information do you think should be included in the expansion board manual?

[cyberman54]

@Bucknalla all GPIOs should be listed, jumper functions, chip types, schematic.