Rollup merge of #152976 - arferreira:revert-relative-extern-paths, r=jieyouxu

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

Author: JonathanBrouwer