docs: Be more vocal about backends different than NVIC

[mchodzikiewicz]

Hi,

I only found out that RTIC already supports some RISC-V targets when I revisited an idea to port it, it would be super cool if documentation could be more vocal about it.

Also, since there are already plenty of backends already, I think it would be very helpful for a broader audience if there was a porting guide for new targets

[Be-ing]

I also did not realize RTIC supports RISC-V from the info on https://rtic.rs and README.md. The description of this repository on GitHub explicitly mentions only ARM. I almost dismissed RTIC as a possibility for my project because I've decided to use an ESP32 MCU, though I haven't decided which specific model yet. If it turns out that RTIC is a good fit for what I'm doing, I would choose one of the RISC-V ESP32 models over Xtensa.