Add Cairo highlighting support straight to mdBook repository

[wawel37]

No description provided.

[maciektr]

After this is done, replace rust with cairo here:

scarb/extensions/scarb-doc/src/docs_generation/markdown/traits.rs

Line 218 in 473afac

writeln!(&mut markdown, "```rust\n{sig}\n```\n").unwrap();

[wawel37]

PR created in mdBook repository. rust-lang/mdBook#2468

[wawel37]

Update: mdBook requires us to make highlight.js support cairo directly. Made PR with changes to highlight.js directly:
highlightjs/highlight.js#4162

[wawel37]

Unfortunately people from mdBook rejected any kind of way of adding support for custom language highlighting as for now. When the mdBook update the highlight.js, we can retry with a Cairo support inside highlighting theme.