cargo: bump mdbook from 0.4.52 to 0.5.1

Bumps [mdbook](https://github.com/rust-lang/mdBook) from 0.4.52 to 0.5.1.
- [Release notes](https://github.com/rust-lang/mdBook/releases)
- [Changelog](https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md)
- [Commits](rust-lang/mdBook@v0.4.52...v0.5.1)

---
updated-dependencies:
- dependency-name: mdbook
  dependency-version: 0.5.1
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]