Document how to configure extra domains

[kvdb]

It took me a while to realize that extra domains to send/receive email from don't need their own mail.extradomain.tld definition, but should just have the MX record point to the FQDN of the mail server.

It's well explained in https://community.mailcow.email/d/2881-multiple-domains-with-separate-mx-entries
It might help other users if this was mentioned in the main documentation.
Perhaps https://docs.mailcow.email/getstarted/prerequisite-dns/#misc would be a good place.