[Event Hubs] Update README to reference service EPH docs

[chradek]

In the event-hubs README we have a note about creating scale-able event consumers and point to our EventProcessor class. Instead, we should point to the service team docs as they provide more context around scaling/load-balancing/checkpointing.

Currently the docs contain .NET-specific code and reference EPH. Once they become language-agnostic we should update.

Current service docs link:
https://docs.microsoft.com/en-us/azure/event-hubs/event-hubs-event-processor-host

[ramya-rao-a]

Can we now link to https://docs.microsoft.com/en-us/azure/event-hubs/event-processor-balance-partition-load?

[ramya-rao-a]

Done with #8599