Add a screenshot of the new "Paste metadata" dialog box

<img width="275" height="217" alt="Image" src="https://github.com/user-attachments/assets/076981d7-2ddf-4e7a-b875-130c6bb1b3e4" />

[This part of the documentation](https://datalab-platform.com/en/features/signal/menu_edit.html#copy-paste-metadata) should mention the new "Paste metadata" dialog box.