Add documentation for work item statuses

What does this MR do and why?

This MR add documentation for work item statuses, which will be generally available in 18.2.

This documentation covers the functionality introduced in Iteration 1 and Iteration 2.

References

• Configurable Work Item Statuses (Main epic)
• Work item statuses - Iteration 1
• Work item statuses - Iteration 2

Screenshots or screen recordings

Not applicable

How to set up and validate locally

Not applicable

MR acceptance checklist

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Related to #527255 (closed)