doc,dns: use code markup/markdown in headers

This will allow us to lint for use of `hostname` in prose without
flagging `hostname` in code within headers. This also allows us to
remove backslash escaping for `[` and `]` inside of header code, which
makes the bare markdown more readable.

Backport-PR-URL: https://github.com/nodejs/node/pull/31109
PR-URL: https://github.com/nodejs/node/pull/31086


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>