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.

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