I have a very old blog post [1] that's basically a build log of the first thermostat I built (plus some rambling). It has a link to the firmware repo and a follow-up post about the API design near the end if you just want to skip to that. I wouldn't really call it "documentation" but it should be enough to point you in the right direction.

I've changed and rebuilt it a few times since then. The version in that blog post expects the microcontroller to have external USB power (I had an outlet with a USB port right next to the thermostat mount, so I didn't care), but you can trivially wire in any voltage converter capable of 24VAC->5VDC/3.3VDC to remove that requirement. I plugged an automotive voltage converter module I had lying around into the first design at some point, and then the next design had a proper rectifier and buck converter to accomplish the same thing.

[1] https://blog.benhaney.com/2019/03/26/building-a-thermostat