Bump mdbook from 0.3.7 to 0.4.0 in /blacksmith

Bumps [mdbook](https://github.com/rust-lang/mdBook) from 0.3.7 to 0.4.0.
- [Release notes](https://github.com/rust-lang/mdBook/releases)
- [Changelog](https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md)
- [Commits](rust-lang/mdBook@v0.3.7...v0.4.0)

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

Author: dependabot-preview[bot]