mgr/dashboard: revise monitoring documentation
The documentation of Prometheus and the Prometheus Alertmanager is hard to read and understand. It is explained that there are several options to configure the Dashboard to receive alerts, of which each provides additional benefits or features. I think it would be better to clearly state which configuration option enables which features.
- Configure the Dashboard "Prometheus receiver" (probably "Alertmanager receiver" would fit better) to receive alerts. This requires to put the URL of the Dashboards' API endpoint into the Alertmanagers' configuration. Alerts are pushed to the dashboard.
- Configure the Alertmanagers API URL in the dashboard. Alerts are pulled.
- Use both. It is taken care of deduplicating alerts.
- Configure Prometheus' API URL to enable auto-completion and validation checks for silences. With the implementation of #42877 this is also required to be able to list all Prometheus configured alerts in the dashboard.