Styling of Rust Frontmatter

[RFC 3503](https://rust-lang.github.io/rfcs/3503-frontmatter.html#reference-level-explanation) adds the concept of a frontmatter to rust for the sake of [RFC 3502: cargo-script](https://rust-lang.github.io/rfcs/3502-cargo-script.html).

Ideally, before stabilization, we decide on the style and get support for it.