Update documentation for serverless-workflow-testing-with-rest-assured example

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### Description

Update documentation for [serverless-workflow-testing-with-rest-assured](https://github.com/apache/incubator-kie-kogito-examples/tree/stable/serverless-workflow-examples/serverless-workflow-testing-with-rest-assured) example.

#### Details
There is adoc reference in the readme file.
adoc file is not aimed to be used directly in the guidelines/readme file. It is just a source that may not be formatted properly.
In addition, the referenced guide is currently in kie repository instead of apache. See https://github.com/kiegroup/kogito-docs/blob/main/serverlessworkflow/modules/ROOT/pages/testing-and-troubleshooting/basic-integration-tests-with-restassured.adoc

### Acceptance criteria
The readme file is updated as follows:
- The readme file should contain the guide that is readable and well formatted.
- The readme file format is one of the following:
- A text directly embedded to the readme file.
- The readme file can be referenced, but it must be accessible, and it is published in apache managed repository/sources.

### Implementation ideas

_No response_

Open the README for the serverless-workflow-testing-with-rest-assured example and compare its current reference with the linked KIE documentation source. Update the README so the guide is readable and well formatted, either embedded directly or linked from an accessible Apache-managed source. Verify that the README meets each acceptance criterion.

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