Add visual marking safe of use pins

[bato3]

I think it is worth following the example of LastMinuteEngineers.com, who explained ESP32 devkit v1 quite thoroughly. And there I really like the section: "Which ESP32 GPIOs are safe to use?", where they added icons showing which pins to avoid and which to watch out for.

Should be marked:

• Not allowed pins,
• Strapping pins
• Not recommended pins (PSRAM, UART, JTAG,...)
• Power Up Glitches pins

[bato3]

related #6

[cgreening]

I'll do that later today along with #6

[cgreening]

Closed by 7e48c07