rustdoc: fix the search-bar layout

Now with broader compatibility.
doc: CSS changes + commit improved favicon
auto merge of #10849 : adridu59/rust/patch-css, r=alexcrichton

rustdoc:
- fix search-bar layout

doc: CSS:
- switch to native pandoc toc depth
- rm some dead code
- clamp width to be readable (we're not Wikipedia!)
- don't background-color titles, it's bloating
- make syntax-highlighting colors inline with rust-lang.org
- space indents

@alexcrichton