since support for kbd elements was added with explicit intent in #175128
it seems like a good idea to support this in nixos-render-docs instead
of just dropping it in favor of `*F12*` etc. since it's a very rare
thing in the manual and purely presentational it makes sense to use
bracketed spans instead of a new myst role.
the html-elements.lua plugin is now somewhat misnamed, but it'll go away
very soon so we don't want to bother renaming it.
