Update mdbook requirement from 0.3.1 to 0.4.2

[dependabot-preview[bot]]

Updates the requirements on mdbook to permit the latest version.

Release notes

Sourced from mdbook's releases.

v0.4.2

See https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-042 for a complete list of changes.

Changelog

Sourced from mdbook's changelog.

mdBook 0.4.2

649f355...9278b83

Changed

• The "show hidden lines" icon has changed from the "expand" icon to an "eye". #1281
• Updated highlight.js. This adds several languages: c, c-like (effectively cpp), csharp (replaces cs), kotlin, less, lua, php-template, plaintext, python-repl, r, scss, typescript. #1277

Fixed

• Fixed SUMMARY links that contained newlines. #1291
• Fixed SUMMARY links that contain %20 spaces. #1293
• Fixed favicon so that if only the png or svg is overridden, the other is not automatically included in the <link> tag. #1272

mdBook 0.4.1

d4df7e7...649f355

Changed

• Removed several outdated dev-dependencies. #1267

Fixed

• Fixed sidebar scrolling if the book includes part titles. #1265
• Don't include the default favicon if only one of the PNG or SVG is overridden. #1266

mdBook 0.4.0

99ecd4f...d4df7e7

Breaking Changes

• Several of the changes in the release have altered the public API of the mdbook library.
• Many dependencies have been updated or replaced. This also removes the --websocket-hostname and --websocket-port from the serve command. #1211
• A new "404" page is now automatically rendered. This requires knowledge of the base URL of your site to work properly. If you decide to use this as your 404 page, you should set the site-url setting in the book configuration so mdbook can generate the links correctly. Alternatively you can disable the 404 page generation, or set up your own 404 handling in your web server. #1221

Commits

• f6768b8 Fix release script LTO.
• 8f7e030 Merge pull request #1299 from ehuss/release-next
• 9180dd1 Release 0.4.2
• 9278b83 Merge pull request #1298 from FabienTregan/adding_info_about_markdown_flavor
• 2674347 Adds information about markdown flavor in the documentation.
• 3d44553 Merge pull request #1293 from Evian-Zhang/master
• 9d5c454 delete '+' replacement and use cargo fmt to format
• a00e7d1 Merge pull request #1272 from ehuss/fix-favicon-link
• 60be20a allow space in SUMMARY.md's link destination
• 8746206 Merge pull request #1284 from ericonr/opt
• Additional commits viewable in compare view

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[dependabot-preview[bot]]

Superseded by #9.