Hugo defaults to serving files with pretty urls [1] - this means
`/posts/foo.org` is served at `/posts/foo/`. This works because servers
default to serving index.html when a directory is specified and hugo renders
the post to `/posts/foo/index.html` instead of `/posts/foo.html`. To make
relative links work we need to (1) remove the fake `foo/` subdirectory from
unrooted links and (2) replace any `.org` suffix with `/`.
[1] https://gohugo.io/content-management/urls/#pretty-urls
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