Add work items to nav and move related pages

Add navigation item to global navigation

This MR adds the new Work items doc to nav (line 1631) and groups the following under it:

• Issues and issue boards
• Tasks
• Epics

This follows the IA plan in gitlab-org&16344 (closed), except pages "Linked items" (that will collect "Linked issues" and "Linked epics") and "Boards" (that will collect "Issue boards" and "Epic boards") haven't been created.

Part of FY26Q1 Stage Lead planning issue: Plan (team-tasks#1252 - closed)

• Link to the merge request that introduced the new page: gitlab-org/gitlab!183847 (merged) | https://docs.gitlab.com/user/work_items/
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page: https://docs.gitlab.com/review-mr-451/user/work_items/ (compare with https://docs.gitlab.com/user/project/issues/).