Support admonitions in option descriptions

[mweinelt]

Good documentation uses notes, warnings, infos, tips sprinkled in to option documentations but sadly s.n.o. currently does nothing to highlight these, because it is missing relevant css classes.

E.g.: https://search.nixos.org/options?channel=unstable&show=services.postfix.config.smtpd_tls_chain_files&from=0&size=50&sort=relevance&type=packages&query=postfix.config

[raboof]

seems like a good idea, we could use some of the styles/colors of https://nixos.org/manual/nixpkgs/unstable/ - anyone up for a PR?