Clearly notify where the on/off rocker switch is on the Glowforge

[benbland]

The Glowforge was unresponsive. I looked around for a power indicator or power switch to see if it was on but couldn't find anything. And there didn't seem to be any notes on this issue in the induction materials on Github. It turns out, there is a power rocker switch hidden from view on the back right-hand-side of the machine, by the power cable socket.

I suggest adding a note in the instructions to clarify this.