Add instructions for adding GitLab EE license to docs nav

Add navigation item to global navigation

• Link to the merge request that introduced the new page: Unknown
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page: https://russell-add-licence-to-navigation.docs.gitlab-review.app/ee/administration/operations/

Notes:

• I know I've misspelled license in the branch name. 😀
• I was unsure just where in the navigation to locate this page. We have a Steps after installing page, and that seemed a good candidate. However, I chose another location because:
  • The "Steps after installing" page already contains a link to the "Install GitLab EE license" page;
  • Installing a license applies to more than just after installing GitLab.