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

[nfrankel]

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

[github-actions]

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[nfrankel]

Keeping the issue alive