rustdoc: clean up source sidebar hide button This is a redesign of the feature, with parts pulled from https://github.com/rust-lang/rust/pull/119049 but with a button that looks more like a button and matches the one used on other sidebar pages. Preview: * http://notriddle.com/rustdoc-html-demo-8/source-sidebar-resize/src/std/lib.rs.html * http://notriddle.com/rustdoc-html-demo-8/source-sidebar-resize/std/index.html | | Before | After | |--|--|--| | Closed |  |  | Open |  |  | Mobile Closed |  |  | Mobile Open |  |