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.
Tror entailed er riktig her, men det hadde kanskje vært bedre med et enklere ord.