doc: Add 'Secure Alertmanager cluster traffic' design document
doc: Add 'Secure Alertmanager cluster traffic' design document

Signed-off-by: Max Leonard Inden <bb17a4a5e30bc4dd6ff6d9ba25e3b87c5052b918@gmail.com>

prometheus/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager,GoogleCloudPlatform/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager