Auto merge of #66966 - ehuss:update-beta-mdbook, r=alexcrichton

[beta] Update mdbook

This is a backport of #66338. There were some significant rendering regressions, particularly in example code, in the version on the beta branch.

This change drops mdbook-linkcheck rather than updating it. The new version brought in a number of new crates and changes that I wasn't comfortable pulling into beta. The linkcheck is only used for the rustc-guide which is not published from this repo.

Detailed notes are at https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md

Author: bors