Clarify expected cardinality between Gateway, Endpoint Selection Extension, and Backend Pod

[robscott]

In #189, @spacewander raised a good concern about the scalability of our systems if multiple extensions were probing the same endpoints for metrics. Although some of us have been working with an assumption that each Pod would be served by exactly one endpoint selection extension, I don't believe we've documented that anywhere. This issue is intended to track broader documentation that covers the expected cardinality for each layer in this system.

/kind documentation