Update mdbook
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.
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