User Help: Add "What is new" item linked to Release Notes

Add "What is new" item to the Help menu:
https://resinsight.org/releases/release-notes/releasenotes_2025_12/index.html

<img width="719" height="331" alt="Image" src="https://github.com/user-attachments/assets/433aebde-43f3-465b-963b-c4f14847b086" />