Docs: Create a "Work items" page

• Start this issue's title with Docs: or Docs feedback:.

Problem to solve

Create a "Work items" page doc/user/work_items/_index.md to:

• (MVC 1) Contain information about the consolidated work items view:
  • Create "Work items" doc (!183847 - merged)
• (MVC 2) Serve as parent page for epics, issues, and tasks pages, per &16344 (closed)
  • Add work items to nav and move related pages (gitlab-org/technical-writing/docs-gitlab-com!451 - merged)
• (in the future) Collect information common for all work item types.

Live URL

https://docs.gitlab.com/user/work_items/

Who can address the issue

@msedlakjakubowski

Other links/references

Docs IA: Reorganize the "Plan and track work" s... (&16344 - closed)