Create a documentation section to use Grafana `DataSource` with SonataFlow Prometheus metrics

[ricardozanini]

Description

After implementing the integration with Prometheus via ServiceMonitor, users now need a way to use the collected metrics. Grafana is ideal for this use case and is commonly used in the industry. We should create a section in our docs explaining how to configure it correctly with the operator.

Prereqs

• Grafana and Prometheus are installed in the cluster
• Grafana CR is already installed and configured in the cluster. The operator won't provision a Grafana server

Depends on https://github.com/apache/incubator-kie-kogito-serverless-operator/issues/465

[jianrongzhang89]

@ricardozanini as we discussed on July 13, for the first implementation of this feature, we will have the SonataFlow operator to create a ServiceMonitor object for each sonataflow if the corresponding flag is set in the platform's spec, and Prometheus has been installed in the cluster. In addition, we decided that the operator will NOT create the prometheus data source in this implementation, and we will provide a documentation on how to create the data source as well as a sample dashboard. We leave the data source and dashboard creation for potential future enhancements based on feedback from users.