docs: Rework the service discovery documentation in general and Eureka in particular

Current State

The Integration service discovery registry mentions service discovery and how you can potentially create your own mechanism. It uses the example for Eureka.

After discussing with @bzp2010, it seems there's no need to create Eureka as it's part of the out-of-the-box features.

Desired State

We shall:

• Document how to use Eureka as a service discovery provider
• Design a new example on how to implement one custom service discovery provider