Remove template code duplication

[Kixiron]

There's some code duplication in templates that's really annoying me

crate/details.html => rustdoc/body.html

[giraffate]

Hi, is the above links broken? Especially in the latter, #L53-L118 is not found now.

[jyn514]

Yes, that part of the codebase had a lot of churn lately. Here's an updated link for rustdoc/header.html: https://github.com/rust-lang/docs.rs/blob/master/templates/rustdoc/header.html#L52-L117

[giraffate]

Thanks for your advice!