Update mdbook requirement from 0.4.43 to 0.5.1

[dependabot]

Updates the requirements on mdbook to permit the latest version.

Release notes

Sourced from mdbook's releases.

v0.5.1

See https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-051 for a complete list of changes.

Changelog

Sourced from mdbook's changelog.

mdBook 0.5.1

v0.5.0...v0.5.1

Changed

• Changed the scrollbar background to be transparent. #2932
• Ignore invalid top-level environment variable config keys. This allows setting things like MDBOOK_VERSION to not cause an error. #2952

Fixed

• Fixed the sidebar heading nav to have the correct nesting levels. #2953
• Various Font Awesome fixes and improvements. #2951

mdBook 0.5.0

v0.4.52...v0.5.0

The 0.5.0 release is the next major release of mdBook, containing over 130 PRs since 0.4.52! The primary focus for this release has been an evolution of the Rust APIs to make it easier to maintain, to evolve in a backwards-compatible fashion, to clean up some things that have accumulated over time, and to significantly improve the performance and compile-times.

This release also includes many new features described below.

We have prepared a 0.5 Migration Guide to help existing authors switch from 0.4.

The final 0.5.0 release only contains the following changes since 0.5.0-beta.2:

• Added error handling to environment config handling. This checks that environment variables starting with MDBOOK_ are correctly specified instead of silently ignoring. This also fixed being able to replace entire top-level tables like MDBOOK_OUTPUT. #2942

0.5 Migration Guide

The 0.5 release contains several breaking changes from the 0.4 release. Preprocessors and renderers will need to be migrated to continue to work with this release. After updating your configuration, it is recommended to carefully compare and review how your book renders to ensure everything is working correctly.

If you have overridden any of the theme files, you will likely need to update them to match the current version.

See the entries below for mdBook 0.5.0-alpha.1, mdBook 0.5.0-beta.1, and mdBook 0.5.0-beta.2 for a more complete list of changes and fixes.

The following is a summary of the changes that may require your attention when updating to 0.5:

Major additions

• Added sidebar heading navigation. This includes the output.html.sidebar-header-nav option to disable it. #2822
• Added support for definition lists. These are enabled by default, with the option output.html.definition-lists to disable it. See docs for more. #2847
• Added support for admonitions. These are enabled by default, with the option output.html.admonitions to disable it. See docs for more. #2851
• Links on the print page now link to elements on the print page instead of linking out to the individual chapters. #2844

... (truncated)

Commits

• d63aeb6 Merge pull request #2955 from ehuss/bump-version
• eb83d08 Update to 0.5.1
• 710ec27 Merge pull request #2932 from vitorfloriano/scrollbar-bg-transparent
• 5d64d0e Merge pull request #2953 from ehuss/fix-heading-nav-level
• 718ceec Fix heading nav depth
• 09616e3 Add a test to validate the structure of the heading nav
• 7e7a3b4 Merge pull request #2952 from ehuss/ignore-top-env-keys
• d39deca Ignore invalid top-level environment variable config keys
• 8385e75 Merge pull request #2951 from ehuss/fa-improvements
• 8571d70 Update the documentation for Font Awesome
• Additional commits viewable in compare view

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