Edits to GitLab Kubernetes Agent doc

What does this MR do and why?

Describe in detail what your merge request does and why. This doc was helpful after reading it from beginning to end. I believe it can also be helpful in a stepwise fashion, with some section numbering that is in line with the summary of steps at the beginning.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.