doc: add webfonts and tweak the styles accordingly
doc: unifying with rustdoc
auto merge of #13587 : adrientetar/rust/more-docs, r=brson

- Use Fira Sans for headlines, Heuristica for the body (Adobe Utopia derivative). Both are licensed under the SIL OFL license. (I didn't include BoldItalic because it is sparingly used.)
- Split TOC into 2 columns for the docs except manual (too tall, too wide to be readable).
- Some fixes to rustdoc, bring styles in coherency with eachother
- A few tweaks

Two examples: [modified tutorial](http://adrientetar.legtux.org/cached/rust-docs/tutorial.htm) and [modified manual](http://adrientetar.legtux.org/cached/rust-docs/manual.htm).

(Rustdoc is pretty much the same but if you want to see it, here is [modified `std`](http://adrientetar.legtux.org/cached/rust-docs/std.htm) and [modified `std::io`](http://adrientetar.legtux.org/cached/rust-docs/io.htm).)

#13484, #13485 follow-up.

cc @brson