chore: add nus wellbeing day to holidays (#4045)

* Update 2025 public holidays

* Add NUS wellbeing day into holidays

Author: jloh02

website/src/data/holidays.json

@@ -69,6 +69,7 @@
   "2025-06-07",
   "2025-08-09",
   "2025-10-20",
+  "2025-10-21",
   "2025-12-25",
   "2026-01-01"
 ]