Rollup merge of #71645 - ecstatic-morse:readme-build-doc, r=Mark-Simulacrum

Rollup merge of #71645 - ecstatic-morse:readme-build-doc, r=Mark-Simulacrum

Direct contributors to try stage 0 rustdoc first

After #71458, `./x.py doc --stage 0 src/libstd` is (empirically) able to build the standard library docs using the `rustdoc` packaged with the bootstrap compiler. This means that new contributors don't need to build the compiler to locally inspect small documentation fixes. This was a roadblock for me when I first started contributing to rust and something that still regularly annoys people. We should recommend that contributors give bootstrap `rustdoc` a try before building the whole compiler.