Update documentation for Pi Setup

[efcy]

• Define a exact version of the raspbian OS. https://downloads.raspberrypi.org/raspbian/images/
• update the wireless setup part. It seems to be wrong
• make it more clear how and why we need to set a static ip for ethernet. Also mention that the static ip should be the ip that is on the label of the pi
• Move documentation to https://github.com/BerlinUnited/berlinunited.github.io

There are probably more things that should be made more clear in the setup