Unrolled build for #142076

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](#141272 (comment)) `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``

Author: rust-timer

src/ci/docker/host-x86_64/mingw-check-2/Dockerfile

@@ -30,6 +30,7 @@ ENV SCRIPT \
         python3 ../x.py check && \
         python3 ../x.py clippy ci && \
         python3 ../x.py test --stage 1 core alloc std test proc_macro && \
+        python3 ../x.py doc --stage 0 bootstrap && \
         # Build both public and internal documentation.
         RUSTDOCFLAGS=\"--document-private-items --document-hidden-items\" python3 ../x.py doc --stage 0 compiler && \
         RUSTDOCFLAGS=\"--document-private-items --document-hidden-items\" python3 ../x.py doc --stage 1 library && \