Add compute minutes admin doc for dot com to the nav

Add navigation item to global navigation

• Link to the merge request that introduced the new page: Document how cost factors can be changed on git... (gitlab-org/gitlab!185357 - merged)
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page: https://docs.gitlab.com/review-mr-488/administration/cicd/dot_com_compute_minutes/