Update to mdbook 0.5

[ehuss]

This updates from mdbook 0.4.43 to mdbook 0.5.1.

The 0.5 release includes a large number of changes. Additionally there were quite a few changes between 0.4.43 and 0.4.52. If you want all the details, see the changelog which includes a 0.5 migration guide: https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-051

[ehuss]

This is blocked by https://github.com/rust-lang/rustup/pull/4612 and https://github.com/rust-lang/rustup/pull/4613 which needs to merge to main and stable first.

[ehuss]

This will be blocked until the next release that contains https://github.com/rust-lang/rustup/pull/4612. I'm happy to let this sit for however long that takes (there's no urgency here).

[djc]

Converting this to draft to make it clear that this should not be merged yet.