Operator Docs: Clarify xsite docs

describe that XSite need the operator to work properly and show within the doc how it works

XSiteRepl DG8.1 is done with a single site master the "xsite-service" will forward all requests to the JGroups coordinator

the site-master is choosen by operator and the jgroups coordinator is used
operator will query all pods and tag the pod where coordinator run

if the coordinator dies, the operator finds the new coordinator/pod and tags that one. the service automatically updates and starts using the new pod