doc: separate manpage URLs from the Pandoc filter

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.

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