tools/gen-cpydiff: Use case description as 3rd-level heading.
tools/gen-cpydiff: Use case description as 3rd-level heading.

This is required to easily giving links to a particular difference case.
Also, add RST anchors to allow cases to cross-reference each other.

swegener/micropython,blazewicz/micropython,bvernoux/micropython,micropython/micropython-esp32,swegener/micropython,pfalcon/micropython,pozetroninc/micropython,chrisdearman/micropython,trezor/micropython,dmazzella/micropython,Timmenem/micropython,ryannathans/micropython,lowRISC/micropython,kerneltask/micropython,dmazzella/micropython,henriknelson/micropython,kerneltask/micropython,tralamazza/micropython,MrSurly/micropython,micropython/micropython-esp32,oopy/micropython,adafruit/circuitpython,TDAbboud/micropython,pramasoul/micropython,swegener/micropython,TDAbboud/micropython,bvernoux/micropython,AriZuu/micropython,pfalcon/micropython,henriknelson/micropython,AriZuu/micropython,henriknelson/micropython,oopy/micropython,AriZuu/micropython,pfalcon/micropython,infinnovation/micropython,adafruit/circuitpython,selste/micropython,ryannathans/micropython,dmazzella/micropython,adafruit/circuitpython,SHA2017-badge/micropython-esp32,bvernoux/micropython,henriknelson/micropython,alex-robbins/micropython,ryannathans/micropython,PappaPeppar/micropython,MrSurly/micropython,kerneltask/micropython,HenrikSolver/micropython,selste/micropython,infinnovation/micropython,HenrikSolver/micropython,AriZuu/micropython,pozetroninc/micropython,pfalcon/micropython,tobbad/micropython,TDAbboud/micropython,trezor/micropython,lowRISC/micropython,adafruit/circuitpython,adafruit/micropython,swegener/micropython,trezor/micropython,blazewicz/micropython,SHA2017-badge/micropython-esp32,pramasoul/micropython,alex-robbins/micropython,infinnovation/micropython,ryannathans/micropython,oopy/micropython,tobbad/micropython,PappaPeppar/micropython,adafruit/circuitpython,torwag/micropython,lowRISC/micropython,adafruit/micropython,Timmenem/micropython,tobbad/micropython,deshipu/micropython,pramasoul/micropython,infinnovation/micropython,kerneltask/micropython,PappaPeppar/micropython,SHA2017-badge/micropython-esp32,pramasoul/micropython,alex-robbins/micropython,AriZuu/micropython,MrSurly/micropython-esp32,PappaPeppar/micropython,MrSurly/micropython,trezor/micropython,swegener/micropython,adafruit/micropython,MrSurly/micropython-esp32,ryannathans/micropython,selste/micropython,deshipu/micropython,selste/micropython,blazewicz/micropython,torwag/micropython,Timmenem/micropython,tobbad/micropython,bvernoux/micropython,chrisdearman/micropython,kerneltask/micropython,MrSurly/micropython-esp32,torwag/micropython,MrSurly/micropython,torwag/micropython,tralamazza/micropython,TDAbboud/micropython,Timmenem/micropython,HenrikSolver/micropython,tralamazza/micropython,pfalcon/micropython,bvernoux/micropython,blazewicz/micropython,pozetroninc/micropython,lowRISC/micropython,lowRISC/micropython,blazewicz/micropython,adafruit/circuitpython,micropython/micropython-esp32,infinnovation/micropython,deshipu/micropython,chrisdearman/micropython,pramasoul/micropython,alex-robbins/micropython,TDAbboud/micropython,torwag/micropython,tobbad/micropython,SHA2017-badge/micropython-esp32,pozetroninc/micropython,deshipu/micropython,PappaPeppar/micropython,oopy/micropython,alex-robbins/micropython,henriknelson/micropython,Timmenem/micropython,tralamazza/micropython,MrSurly/micropython,adafruit/micropython,dmazzella/micropython,selste/micropython,HenrikSolver/micropython,HenrikSolver/micropython,pozetroninc/micropython,chrisdearman/micropython,oopy/micropython,chrisdearman/micropython,deshipu/micropython,adafruit/micropython,MrSurly/micropython-esp32,trezor/micropython,MrSurly/micropython-esp32,micropython/micropython-esp32,SHA2017-badge/micropython-esp32,micropython/micropython-esp32