No need to depend on a third-party hook repository when each of these
checks is easily defined and run through system commands.
This also allows us to actually run tests, which is current unsupported
(https://github.com/doublify/pre-commit-rust/pull/19)
This removes mdBook in favor of a flat docs folder generating a single
`README.md` in the repository root.
Installation and usage instructions have also been expanded slightly.
