Rollup merge of #133636 - GuillaumeGomez:sans-serif, r=rustdoc

[rustdoc] Add sans-serif font setting

Fixes rust-lang/rust#52449.

This PR adds a new setting introducing the possibility to switch to a sans-serif font (`Fira Sans`) for the text.

Can be tested [here](https://rustdoc.crud.net/imperio/sans-serif/std/index.html).

cc ```@rust-lang/rustdoc-frontend```
r? ```@notriddle```