Improve build instructions

[MJohnson459]

I mostly followed the ORP rules when considering how to attach the components to the board. This works but feels sub-optimal in some places. To use the space available, we need to optimise the shapes of some mounts to position them on the edge. In other cases, 20mm spacing is just not enough manoeuvrability so we have mount spacing of 10mm to allow more play.

I also found that tightening the nuts was awkward so in most of the mounts I added hex cutouts. This allows the nuts to stay in place while the bolt is tightened. However this enforces an order and direction on the connections which can be problematic, i.e. if the screw used to mount the Raspberry Pi is even slightly too long it might damage the board.

There are a few directions we could try:

1. Completely rethink or at least extend the ORP rules. Maybe a finer grid, or a second offset grid would allow more flexibility without impacting strength. Maybe a hex grid if its possible to do easily in CAD.
2. Force a connection direction of top down. This would make it simpler to build as you don't need to go back on yourself. It implies the bolts would always be inserted from the top and so the nut must be below. We would need to redesign a lot of components and possibly add hex cutouts to the bottom of the chassis. This might reduce flexibility in which bolts can be used.
3. Replace bolts with some sort of clip system? We would need to be careful ensuring it can be deconstructed easily and that the printed clips are strong enough.