Update docs with new Group Runners location

What does this MR do and why?

Minimum viable change for docs, to showcase the new Runner Groups view.

Follow up of !82622 (comment 872139729)

Screenshots or screen recordings

Changes will be visible at: https://docs.gitlab.com/ee/ci/runners/runners_scope.html

How to set up and validate locally

NA

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.

Related to #19819 (closed)