Add documentation of Parent widget for tasks and OKRs

What does this MR do and why?

This MR adds documentation of Parent widget for tasks and OKRs

Screenshots or screen recordings

No visual changes.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #421611 (closed)