Rollup merge of rust-lang#66338 - ehuss:update-mdbook, r=alexcrichton

Update mdbook.

This brings in some important updates to fix some rendering issues in the books. In particular fixing hidden lines in code blocks, and some escaping issues.  More details at https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md

This also requires updating mdbook-linkcheck.

Author: JohnTitor