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The Electronics Stack Exchange has a built-in button to a free version of Circuit lab. It's not a bad little tool for creating simple circuit diagrams. I don't see a way to do that in the Arduino Stack exchange, at least not directly. I had to cheat, create a throw-away question in the EE Stack exchange, export it to a picture, then post THAT, which is quite convoluted.
If not, could Stack Exchange add that feature to the Arduino boards? It would be very useful and appropriate.