Add README for custom 404 widget

Added usage instructions and use case for custom 404 widget.

Author: raghavs046

Modern Development/Service Portal Widgets/custom404/README.md

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+**How to use**
+1. Add this widget to new portal page.
+2. Add the page in "sp_portal" record in 404 field.
+
+**Use Case**
+1. Some organizations do not want to show OOB 404 page having game but want to show the suggestions with correct pages.
+
+**Result**
+1. This code will search the 3 letters of page_id from URL and will suggest correct pages.
+2. The correct links will open the pages in next tab.
+
+   <img width="959" height="413" alt="404" src="https://github.com/user-attachments/assets/2480ba26-4ea3-4e12-baf8-bbf89fec548a" />
+
+
+