Rollup merge of rust-lang#119492 - rustbot:docs-update, r=ehuss

Update books

## rust-lang/reference

1 commits in f9f5b5babd95515e7028c32d6ca4d9790f64c146..3565c7978cfc9662f5963b135690ff9cbbfa0318
2023-12-29 21:01:19 UTC to 2023-12-29 21:01:19 UTC

- Use proper footnote for number literals `_` separator note (rust-lang/reference#1444)

## rust-lang/rust-by-example

1 commits in 4c2b24ff9d9cf19f2fcff799a3a49b9a2c50ae8e..c0be6299e52e4164c30ba6f41bd0ad0aaee64972
2023-12-21 15:22:11 UTC to 2023-12-21 15:22:11 UTC

- point to raw indentifiers, talking about fields (rust-lang/rust-by-example#1789)

## rust-lang/rustc-dev-guide

5 commits in 0610665..d13e851
2023-12-30 09:58:25 UTC to 2023-12-19 10:02:34 UTC

- Prominently mention `profiler = true` on the coverage page (rust-lang/rustc-dev-guide#1844)
- Add a description of `unpretty=hir` to the HIR docs (rust-lang/rustc-dev-guide#1842)
- Fix typo in unsize docs (rust-lang/rustc-dev-guide#1843)
- Suggest `gcc_multi` to make `mir_opts` run on nixos (rust-lang/rustc-dev-guide#1841)
- bootstrapping: Clarify that stage0 std code is not executed to produce stage1 compiler (rust-lang/rustc-dev-guide#1840)