doc: allow <code> in header elements

Allow use of <code> in header elements without styling side effects. We
can add styling later if desired. The goal here is to allow code to be
set off in markdown without needing to escape characters and do lint
exceptions for terms. This is probably a win in terms of accessibility
too although it would be moreso if we had some visual differentiation
for <code> inside of headers. As mentioned above, that can always be
added at a later time.

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>