Update mdbook

[ehuss]

This updates mdbook to 0.4. The list of changes can be found at https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-040. I think the most important one is the change to include fonts with the book instead of using the Google Fonts CDN. This adds a few megabytes of fonts to the docs component. It may be possible to share the fonts across the books, but would take a fair bit of work to make that happen, so I'm not sure if it is necessary.

This also removes mdbook-linkcheck. It is currently not being used, and I don't foresee it going back into use anytime soon. I would prefer not to maintain something that isn't being used, and it removes a very large number of dependencies.

[rust-highfive]

r? @Mark-Simulacrum

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[bors]

☔ The latest upstream changes (presumably #73756) made this pull request unmergeable. Please resolve the merge conflicts.

[Mark-Simulacrum]

Hm, so with the google fonts change -- I am a bit worried about redistributing those in our artifacts. Do we know if the licensing allows for that? Do we need some call out? Maybe we should disable copying the fonts for now and deal with it later (or never).

[ehuss]

I'm pretty sure the license is OK. Open-Sans is apache and source code pro is OFL which explicitly allows redistributing. The licenses are also included in the distribution. I don't think there is any requirement for attribution. Rustdoc already copies/embeds source-code-pro (among other fonts).

[Mark-Simulacrum]

Okay, seems good, thanks for checking. @bors r+

[bors]

📌 Commit 6bcbc42 has been approved by Mark-Simulacrum

[Mark-Simulacrum]

I'm actually going to @bors rollup=never this as well because it seems like we'd want to be able to bisect accurately just in case

[Dylan-DPC-zz]

@bors p=1

[bors]

⌛ Testing commit 6bcbc42 with merge 9bdd2db...

[bors]

☀️ Test successful - checks-azure
Approved by: Mark-Simulacrum
Pushing 9bdd2db to master...