Update info re supported Alertmanager configurations

1. Proposed title of this feature request

To Have Supported Alertmanager configuration written in OpenShift Official Document.

2. What is the nature and description of the request?
Request is add all supported Alertmanager configuration in OpenShift Official Doc.

3. Why does the customer need this? (List the business requirements here)

Hello Team,

I have a customer who is requesting to add the supported alertmanager configuration in our official document. If you refer to link[1], it states See Alertmanager configuration for configuring alerting through different alert receivers which redirects to the Prometheus document link. Since OCP 4.8 ships Alertmanager v0.21 so this page applies https://prometheus.io/docs/alerting/0.21/configuration/ .

CU is concern Because https://prometheus.io/docs/alerting is a page which hasn't been managed by Red Hat, this page can be removed or modified suddenly.Red Hat support team always says that only things written in OpenShift manual are supported officially.

Previously CU faced a problem one of  VMware's page, which had been linked from OpenShift installation guide, was suddenly removed.They do not want to face same issue.

[1]:https://docs.openshift.com/container-platform/4.8/monitoring/managing-alerts.html#applying-custom-alertmanager-configuration_managing-alerts