Update standoffs in PCB assembly documentation and BOM

[apollokit]

Figure out what standoffs to actually use for the PCB, and update the BOM and pcb build readme

Let's go with all metric, to make things easier for the future

[Achllle]

I'm using:

• 30mm standoffs for the motor board to case connection
• 20mm standoffs for the RPi on the brain board
• 15mm for the Roboclaws
• 10mm for the brain board to motor board connection

I believe these are M2.5 standoffs (the RPi only does M3 if you drill them out). I got some standoff kit on Amazon.