Pcb section rework

Moves the 'physical overview' part from system overview to pcb.
Also sets up a sugggestion for the pcb testing part of the 'Tests' section.
Lastly, also includes a couple of sentence changes here and there, but someone should look at them and see what they think before merging.
Entailed virker ikke helt som riktig ord. Forslag til endring:  For the soldering tests we checked that the connections held, and that the power propagated properly though the board. Er bare et forslag da,  men iaf bruke et annet ord enn entailed.