Reorder epics and issues docs in nav

Add navigation item to global navigation

Reorder Work items doc sections to flow more logically and list Epics > Issues > Tasks, from the highest-level planning object to the lowest.

Before:

• Work items
  • Issues +
  • Tasks
  • Epics +
  • Linked items

After:

• Work items
  • Epics +
  • Issues +
  • Tasks
  • Linked items

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• Link to the merge request that introduced the new page:
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page:

Docs: Create a "Work items" page (gitlab-org/gitlab#523418 - closed) | Docs IA: Reorganize the "Plan and track work" s... (gitlab-org&16344 - closed)