Draft: Use 'runner configuration' name when appropriate

What does this MR do and why?

This MR tries to make it clearer when we're referring to runners vs 'runner configurations'. It should help reduce confusion among users, since we currently use the concepts interchangeably in some places, due to the legacy documentation.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

• Before: https://docs.gitlab.com/ee/ci/runners/new_creation_workflow.html#the-new-runner-registration-workflow
• After: https://main-ee-131163.docs.gitlab-review.app/ee/ci/runners/new_creation_workflow.html#the-new-runner-registration-workflow

How to set up and validate locally

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

MR acceptance checklist

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