feat(dns): add instructions for customizing DNS configuration template

[jijiechen]

Kuma users can now override DNS configuration template from both control plane and data planes. So this PR is to add instructions to implement it.

Please also see issue for the underlying feature: https://github.com/kumahq/kuma/issues/8119

Did you sign your commit? Instructions Yes.

Have you read Contributing guidelines? Yes.

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[lahabana]

Should we add in the example this one to disable AAAA answers:

.:{{ .CoreDNSPort }} {
    template IN AAAA . {
       rcode NOERROR
       fallthrough
    }
    forward . 127.0.0.1:{{ .EnvoyDNSPort }}
    # We want all requests to be sent to the Envoy DNS Filter, unsuccessful responses should be forwarded to the original DNS server.
    # For example: requests other than A, AAAA and SRV will return NOTIMP when hitting the envoy filter and should be sent to the original DNS server.
    # Codes from: https://github.com/miekg/dns/blob/master/msg.go#L138
    alternate NOTIMP,FORMERR,NXDOMAIN,SERVFAIL,REFUSED . /etc/resolv.conf
    prometheus localhost:{{ .PrometheusPort }}
    errors
}

.:{{ .CoreDNSEmptyPort }} {
    template ANY ANY . {
      rcode NXDOMAIN
    }
}

This is one of the main reason to add this feature