Add a 12V rail for the N64 slot. This is not currently implemented, but as the six-slot adapter is the hardest of the 2 boards to upgrade having it now will save some effort for anyone wanting it in the future. It also makes it easier to externally supply 12V by attaching clips to the 12V and GND header pins.
Also improved the silkscreen a bit to match the main PCB.
* Add JLCPCB order ID text to Six-Slot Adapter; you can select "specify location" for order number location when ordering from JLCPCB. It is placed so the SNES slot covers it. (PCB revision not incremented because it is only a minor change to the silkscreen.)
* Added relative path for footprint and symbol libs to six-slot adapter project files.
* Added footprint details/links to schematic symbols.
* Added .gitignore and included most of the files that should remain local-only.
