Integration reference on Docs

[siavashs]

Currently we reference integrations in the configuration page https://prometheus.io/docs/alerting/latest/configuration/

Some integrations are named in different pages of the docs as examples but we do not have a complete list.

We need a list or table which includes:

• all supported integrations
• link to Alertmanager configuration section
• link to external documentation
  • external service configuration docs if any, otherwise the integration home page
  • api reference

This will help users to more easily find what is supported or not and quickly access configuration references.
All existing and new integrations must be listed there.

We can also have a dedicated section about adding new integrations, including our policies and maintenances, etc.