rustdoc: add webfonts and tweak the styles accordingly

- Cherry-pick from #12996
- Use Fira Sans for headlines and sidebar (Light), Heuristica for the body (Adobe Utopia derivative). Both are licensed under the SIL OFL license.
- A few tweaks

Two examples: [modified `std`](http://adrientetar.legtux.org/cached/rust-docs/std.htm) and [modified `std::io`](http://adrientetar.legtux.org/cached/rust-docs/io.htm).

cc #13484
**Blocked on graydon/rust-www#25 (for hosting of the fonts), that's showcased [here](http://adrientetar.github.io/rust-www/).**

cc @brson, @TheHydroImpulse