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

### Current State

The [Integration service discovery registry](https://apisix.apache.org/docs/apisix/discovery/) 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

Start with the linked service discovery documentation at https://apisix.apache.org/docs/apisix/discovery/ and compare its current Eureka example with the desired state. Done means the documentation explains how to use Eureka and includes a new example showing how to implement a custom service discovery provider.

Written by the indexing model from the issue text.