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.
Until now, this script has used the version of pandoc from unstable.
This means that running the script on the same version of Nixpkgs
could produce different results, and meant that when Pandoc's output
was changed, random PRs were changing the whole manual when they ran
the script to regenerate docs[1][2].
Here I've changed the manual to use a consistent version of pandoc —
the one from the latest release tag, which will avoid this problem in
future. This will avoid this problem in future. The only time we'll
need to worry about pandoc output changes is when we bump the version
used in this script.
I also considered using the version from the current Nixpkgs branch,
but decided against it as it's unlikely that e.g. the person bumping
Pandoc will remember to regenerate the manual.
[1]: https://github.com/NixOS/nixpkgs/pull/162550
[2]: https://github.com/NixOS/nixpkgs/pull/168535
