Remove unneeded nightly features and improve documentation

[Alonely0]

This PR removes some unnecessary feature flags that were stabilized or easily replaceable, and has merged the scattered module-level documentation across the repo.

[ihciah]

Thanks for your contribution! The reason for adding #![allow(stable_features)] is to provide partial compatibility for old nightly toolchains(indeed some of them can be removed).

[Alonely0]

Thanks for your contribution! The reason for adding #![allow(stable_features)] is to provide partial compatibility for old nightly toolchains(indeed some of them can be removed).

Oh, fair enough. It's fixed now. Can you run the CI again? I have no idea why it is failing, it works on my machine.

[ihciah]

I have no idea why it is failing

Nerver mind, we can fix it later. Thanks!