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.
matching on only `{...}` does not trigger if the role tag is preceded by
something usually considered a semantic separator that isn't a separator
as markdown knows it, e.g. punctuation characters.
Move the manpage-to-URL mapping to `doc/manpage-urls.json` so that we can
reuse that file elsewhere, and generate the `link-manpages.lua` filter from
that file.
Also modify the Pandoc filter so that it doesn't wrap manpages that are
already inside a link.
Keeping a Lua filter is essential for speed: a Python filter would
increase the runtime `md-to-db.sh` from ~20s to ~30s (but Python is not
to blame; marshalling Pandoc types to and from JSON is a costly operation).
Parsing in Lua seems tedious, so I went with the Nix way.
