Rollup merge of #142076 - Kobzol:doc-bootstrap-ci, r=marcoieni

Check documentation of bootstrap in PR CI

It's annoying when wrong doc comments in bootstrap [break](https://github.com/rust-lang/rust/pull/141272#issuecomment-2943614152) `auto` CI. This has happened a few times recently, and documenting bootstrap with the stage0 compiler should be pretty quick, so let's add it to PR CI.

r? ``@marcoieni``