add 25 anni challenge to navbar (#1624)

Adds the redirect for the anniversary challenge to the navbar, so people
can discover it - based on community feedback.

Co-authored-by: Nikoś <nikoshell20@gmail.com>

Author: dariagrudzien

src/data/links.json

@@ -99,6 +99,10 @@
           "name": "FAQ",
           "path": "/faq"
         },
+        {
+          "name": "Anniversary Challenge",
+          "path": "https://ep2026.europython.eu/25anniversary"
+        },
         {
           "name": "Code of Conduct",
           "path": "https://www.europython-society.org/coc/"