Document the /status URL for Confluence

Confluence 5.8 added a new endpoint that can be accessed at http://<base_url>/status which displays a simple output when it is running:

{"state":"RUNNING"}

This URL is useful for monitoring systems to quickly check whether Confluence is up and running. This works in both Server and Data Center versions, but is especially useful in multi-node Data Center deployments so that a load balancer knows when remove a particular node from the pool.

We should add documentation for this URL for Confluence as it has been done for JIRA DC (under "Load Balancing Health Check URL").