Libstd should set doc root to point to stable docs for stable releases

[bluss]

Libstd defines html_root_url = "https://doc.rust-lang.org/nightly/", in its crate root. This should change to point to stable docs for stable releases of rust, so that stable rust/rustdoc generate pages that link to those docs.

[arcnmx]

Nitpick that probably doesn't matter: https://doc.rust-lang.org/stable/ vs https://doc.rust-lang.org/1.5.0/ is somewhat of a concern. While using stable is the better option, "stable" doesn't necessarily imply that it will be semver-compatible forever if a Rust 2.0 is ever released!

[brson]

A potential solution: #19603

[dtolnay]

As noted in #35307 (comment), this needs to be automated in a way that does not require a PR like #35307 before every release. I would welcome a PR to solve this permanently if anyone has ideas for how. I don't believe we need an RFC.

[kennytm]

Does something like #![html_root_url = concat!("https://doc.rust-lang.org/", env!("RUST_CHANNEL"))] work? IIRC it doesn't (otherwise we would have #[doc = include_str!(...)] already)