Update doc/user/clusters/agent/ci_cd_workflow.md

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR update the CI-CD Kubernetes impersonation documentation to contain a little more guidance on implementing and using this feature in GitLab. A customer logged a ticket for assistance after failing to implement by following the documentation. When we tried the same we also could not get it to work as we were not aware needing to setup the RBAC ClusterBinding in Kubernetes. This documentation may clarify this requirement and prevent future rookie mistakes like this.

Additionally I have re organised things a little as it make more sense in my honest opinion.

When we at support tried replicating a customer ticket this in our test environment by following the documentation we were not successful.

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None, this is documentation only.

How to set up and validate locally

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