Document how cost factors can be changed on gitlab.com

What does this MR do and why?

Documents the gitlab.com only functionality of changing the cost factor by editing the runner. Adds in a new page for gitlab.com administration of minutes so that it's available for team members to understand how cost factors can be set by admins but not distracting for customers.

See code:

• https://gitlab.com/gitlab-org/gitlab/-/blob/52909b088eb0426a66d9e4eee398b8cec0240068/app/controllers/admin/runners_controller.rb#L58
• https://gitlab.com/gitlab-org/gitlab/-/blob/52909b088eb0426a66d9e4eee398b8cec0240068/ee/app/assets/javascripts/ci/runner/components/runner_update_cost_factor_fields.vue#L52-82

Screenshots or screen recordings

How to set up and validate locally

1. visit gitlab-host/admin/runner
2. Click to edit a runner
3. Observe no cost factor fields
4. in a terminal window navigated to the gitlab or gdk dir
   • $ export GITLAB_SIMULATE_SAAS=1
   • $ gdk reconfigure && gdk restart
5. Now reapeat steps 1 and 2
6. Observe cost factor fields
7. Remember to put back $ export GITLAB_SIMULATE_SAAS=0

MR acceptance checklist

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