Review / change the documentation for ZMX/ZIO2

[atooni]

This is to update the documentation of ZMX after the metrics API has moved to ZIO core.

Primarily we would update the content, but at the same time we might want to update the site to docusaurus 2.

This work would involve:

• Retest and describe the steps to visualize the test metrics in Prometheus / Grafana
• Retest and describe the steps to visualize the test metrics in StatsD / Datadog
• Retest and describt the steps to visualize the test metrics in the developer client
• Describe any other backends we might implement
• Add a best practices section on metrics capturing

[atooni]

Started to document the new ScalaJS client