Rollup merge of #145417 - a4lg:riscv-arch-platform-guide-ch2, r=Amanieu

std_detect: RISC-V platform guide documentation

This is practically a revert of a revert, making the commit e907456b2e10622ccd854a3bba8d02ce170b5dbb on `stdarch` come around again with minor fixes, enhancements and adjustments.

An excerpt from the original commit message follows:

Since there's no architectural feature detection on RISC-V (unlike `CPUID` on x86 architectures and some system registers on Arm/AArch64), runtime feature detection entirely depends on the platform-specific facility.

As a result, availability of each feature heavily depends on the platform and its version.

To help users make a decision for feature checking on a RISC-V system, this commit adds a platform guide with minimum supported platform versions.

Author: jhpratt