Document expected differences in behavior between Sidecar and Ambient modes

[ilrudie]

Per this slack thread, there are some important differences between sidecar and ambient which are difficult for users to understand. In this case it is the differences in what telemetry a sidecar setup emits vs a ztunnel-only setup. User is deploying with flux so install time warnings/notes are not a full solution.

IMO getting the doc started/published soon is more important than trying to exhaustively document. The doc can serve as a jumping off point for improvements laters.

[keithmattix]

+1 - folks who are at least somewhat familiar with Istio may expect all the L7 stuff to exist once they finish installing. We need to help them through this friction point