Problem: "deploy docs" always pulls in the `latest` release of `mdbook`,
which now is a v0.5.0 prerelease with breaking changes -- including
removing an (apparently unused) `multilingual` config field in the TOML
that is now an error (another breaking change).
Solution: Delete the line. Add `workflow_dispatch` to the docs workflow
in case follow-up changes are needed; see
https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#05-migration-guide
