Add usage instructions and use case for widget

Provides instructions on how to use the widget and its use cases.

Author: raghavs046

Modern Development/Service Portal Widgets/My Mentioned Items/README.md

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+**How to use**
+1. Add this widget to portal homepage or any other page.
+2. It will display the top 5 records where user is mentioned in.
+3. It will also display the user who has mentioned the logged in user.
+
+**Use Case**
+1. User will receive the mentioned items on portal homepage.
+2. User can directly go to the record and reply.
+3. The link will land the user on tickets page.
+
+<img width="338" height="957" alt="my mentions" src="https://github.com/user-attachments/assets/68499e33-8d57-4c08-8228-b152203fbf4e" />
+