Rollup merge of #143985 - makai410:rp-rename, r=oli-obk

rustc_public: de-StableMIR-ize

This PR updates relevant docs about StableMIR, basically just rewording StableMIR/SMIR to rustc_public/rustc_public's IR.

The README.md in the `rustc_public` crate is out-dated. I plan to rewrite it after we fork rustc_public into its own repository.

This PR doesn't change the fact that we still use `-Z unpretty=stable-mir` as a rustc parameter for printing the IR, since I feel it's a bit verbose and weird if we use `-Z unpretty=rustc-public-ir`. I was wondering if we can have a short and easy alias for rustc_public's IR.

Author: jieyouxu