Styles are broken on old rustdoc pages

[jyn514]

Probably because of #981.

[jyn514]

Hmm it's not actually related to mobile but to the version of the page. Maybe the rustdoc page structure changed and #930 didn't take the old structure into account?

[jyn514]

I tried removing the duplicate rustdoc-navigation (#1027 (comment)) without success. I think the issue is that vendored.css is missing on old rustdoc pages.