syntax highlight code examples in docstrings
auto merge of #6771 : thestinger/rust/highlight, r=luqmana

This works with pandoc linked against highlighting-kate >= 0.5.3.8. It seems to just be a no-op with earlier versions, because I successfully ran this through `try`.

This also fixes some consistency issues (like making `Example`/`Examples` always a header and always using three tildes).