Add Linked items doc to nav

Add navigation item to global navigation

Live URL: https://docs.gitlab.com/user/work_items/linked_items/

• Link to the merge request that introduced the new page: gitlab-org/gitlab!184827 (merged)
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page:

gitlab-org/gitlab#510355 (closed)