Add Cache-Control header to sample Caddyfile

Basic functionality depends on recent versions of files, although the
fact that every status needs to request the same base index implies
that some level of cacheing is desirable. A cache of one hour is a good
compromise; indices are unlikely to need to be refetched within a
single session, but are likely to be updated between sessions.

For IRI lookups, always consider the file stale to ensure that these
redirects always “work”.