Make source links look cleaner Change from syntaxy-looking [src] to the plain word "source". Change the syntaxy-looking `[-]` at the top of the page to say "collapse". Reduce opacity of rightside content. Part of #59851 r? `@GuillaumeGomez` Demo: https://rustdoc.crud.net/jsha/source-link-2/std/string/struct.String.html [Discussed on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/266220-rustdoc/topic/display.20of.20source.20link).