Document diag-addr and some key endpoints

[klizhentas]

Description

It is worth documenting our diag-addr endpoint as it's used in Kubernetes deployments and prometheus endpoint is used in production to check health of components:

New readyz endpoint is added to existing
/metrics and /healthz endpoints activated by
diag addr-flag:

teleport start --diag-addr=127.0.0.1:1234

Readyz endpoint will report 503 if node or
proxy failed to connect to the cluster and 200 OK
otherwise.

Additional prometheus gagues report connection
count for trusted and remote clusters:

remote_clusters{cluster="one"} 1
remote_clusters{cluster="two"} 1

trusted_clusters{cluster="one",state="connected"} 0
trusted_clusters{cluster="one",state="connecting"} 0
trusted_clusters{cluster="one",state="disconnected"} 0
trusted_clusters{cluster="one",state="discovered"} 1
trusted_clusters{cluster="one",state="discovering"} 0