Auto merge of #33092 - alexcrichton:rustbuild-docs, r=brson

rustbuild: Run all markdown documentation tests

This commit adds support to rustbuild to run all documentation tests, basically
running `rustdoc --test` over all our documentation.

diff --cc src/bootstrap/build/check.rs
Simple merge

diff --cc src/bootstrap/build/mod.rs
Simple merge

diff --cc src/bootstrap/build/step.rs
Simple merge