Revert relative paths for std links in rustc-docs Reverts rust-lang/rust#152243. The relative path `../` passed via `--extern-html-root-url` produces broken links in compiler documentation. The path is wrong in both the published layout (`doc.rust-lang.org/nightly/nightly-rustc/` vs `doc.rust-lang.org/nightly/core/`) and local builds (`compiler-doc/` vs `doc/`). This restores absolute URLs via `#[doc(html_root_url)]`. Fixes rust-lang/rust#152917 Re-opens rust-lang/rust#151496 cc @eggyal