std_detect: RISC-V platform guide documentation

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.

Note:

It intentionally omits the description of the reverse implication related
to *extension groups* (such like implication of `B` *from* its members:
`Zba`, `Zbb` and `Zbs` extensions) because it currently does not synchronize
well with the `-Ctarget-feature` compiler option (due to missing reverse
implication checks using `cfg` and due to constraints of the current Rust's
feature handling).

Instead, it only describes forward implications (like `D` implying `F`) due
to the fact that it relatively synchronizes well between Rust and `stdarch`
for this kind of feature handling (not fully synchronized though).

Still, an extension group is considered "supported" once the
platform/version supports runtime detection of all members in it.

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