This is more notes for me than anything else, but maybe someone else interested in using a Pi to program their Arduino would be interested.
It can be done:
https://learn.adafruit.com/ftdi-friend/programming-the-arduino-bootloader
https://learn.adafruit.com/program-an-avr-or-arduino-using-raspberry-pi-gpio-pins/programming
https://oxygene.sk/2015/02/using-raspberry-pi-as-an-arduino-avr-programmer/
https://learn.sparkfun.com/tutorials/raspberry-gpio/gpio-pinout
https://learn.adafruit.com/introducing-pro-trinket/pinouts
https://github.com/adafruit/Adafruit_ProTrinket_Bootloader
sudo avrdude -p atmega328p -C ~/avrdude_gpio.conf -c pi_1 -v -U flash:w:Blink.cpp.hex:i
