Document about Azure Spring Cloud Starter Event Hubs

[yiliuTo]

The official documentation for Azure Event Hubs only covers azure-spring-cloud-stream-binder-eventhubs, which is to introduce how to use the library with Spring Cloud Stream Framework. However, there is another library azure-spring-cloud-starter-eventhubs under Azure Spring Cloud libraries for Event Hubs, it provides other features like sending / receiving events to / from Event Hubs and integrating with Spring Integration. We should consider to writing another document for it.

The aim of the doc for azure-spring-cloud-starter-eventhubs is to introduce its features and related usage examples, which should contain:

1. Using the starter combined with Spring Integration Framework.
2. Providing simple APIs based on EventHubOperation to send / receive events to / from Event Hubs.
3. Provding an annotation of @AzureMessageListener which can listen to events from Event Hubs.

[saragluna]

Closing this issue now, for we have deprecated this starter, and have the new features in https://aka.ms/spring/docs.