build(deps): Bump mdbook from 0.4.27 to 0.4.28 (#18)

Bumps [mdbook](https://github.com/rust-lang/mdBook) from 0.4.27 to 0.4.28.
- [Release notes](https://github.com/rust-lang/mdBook/releases)
- [Changelog](https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md)
- [Commits](rust-lang/mdBook@v0.4.27...v0.4.28)

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>