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