Add "+, 1" and "-, 2" markings to DC-DC (for soldering)

[CRImier]

During assembly, it's easier if the user first solders the input pin of the DC-DC to the back board (as it needs less heat to solder and therefore will be easier to re-align). So, we can add "+, 1" and "-, 2" markings on the silkscreen to signal the pin order (and, possibly, also somewhat help people that might use a custom DC-DC instead of the recommented one)