* docs: Add example for customizing IMAP folders (mailboxes)
* chore: Update `15-mailboxes.conf` to sync with upstream
This config has not been updated since 2016 (ignoring the Junk autosubscribe addition).
Synced to upstream equivalent at https://github.com/dovecot/core/blob/master/doc/example-config/conf.d/15-mailboxes.conf
Retains the `Archive` example definition from this PR and prior `auto = subscribe` additions.
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Co-authored-by: Brennan Kinney <5098581+polarathene@users.noreply.github.com>
