Add tutorial about converting a personal namespace to left nav

Add navigation item to global navigation

• Link to the merge request that introduced the new page: gitlab!107821 (merged)
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page: https://385281-add-tutorial-to-left-nav.docs.gitlab-review.app/ee/tutorials/convert_personal_namespace_into_group.html