WIP: Resolve "Docs feedback: Run runner on kubernetes"

What does this MR do?

Introduce clarification into the documentation page for "Run a runner on kubernetes". It was previously not clear how to use this documentation, specifically where to get the runner token which is different from the registration token.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/57265

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

Closes #57265 (moved)