Style frontmatter according to the style guide

[epage]

rust-lang/rust#145617 specifies the behavior for the unstable frontmatter as directed in rust-lang/style-team#212

Support for this still needs to be implemented. rust-lang/rust#145766 made sure that rustfmt doesn't eat the frontmatter, at least. This is blocked on a submodule update.

[calebcartwright]

Thanks Ed

Just acknowledging receipt and noting it's a top priority. Will be reachable in Zulip if needed