Sorry, there are no overview docs except the readme file. But for RRF3 on SAME70XPLD, if you are using the latest source code on the -v3-dev branch then you shouldn't need to change anything except the Pins.h file. Use the version of it for Duet3_V05 as a model. Everything else is configured using GCode in the config.g file on the SD card.
It helps if the STEP pins to the drivers are all on the same 32-bit MCU port. But you probably won't have that choice as you are using off-the-shelf boards. So you will need to change the 3 functions at the end of Pins.h. For guidance on how to do this, take a look at the same 3 functions at the ends of Platform.h in the dev branch of RRF, in particular how they were implemented for Radds/Due.
When I built the SAME70XPLD configuration a few days ago, it compiled with no errors, just a few warnings caused by the pin table being empty.
HTH David