I don’t think they’ve started documenting this yet. No offense, but generally, people that want to add custom peripherals would understand how to get started with necessary code. So it’s not surprising if existing devices is prioritized in documentation.
First of all, you should look at libdaisy sources - SPI handler, its usage for OLED display
The sample SPI DMA project from STM may also shed some light (same MCU family, but not identical).
Perhaps you can get a better example from one of the developers.