Rollup merge of #120403 - seqre-contrib:pre-vendored-message, r=onur-ozkan

Add instructions of how to use pre-vendored 'rustc-src'

This PR closes #110163.

I had to move the URL to the left, making it not aligned as it is three lines above, but the tidy check would yell at me otherwise. If that's not acceptable, I'd love some suggestions on how to make it better.

One question: in the original issue (rust-lang/rust#110163 (comment)), it was suggested to mention how to download specific commit tarballs; however, it was said it's not documented anywhere, so I did not include that yet. If there is a want to have that, I'd gladly amend the commit.

Author: GuillaumeGomez