POST IA: Fix redirect old custom-signup-signin-pages paths (#2643)

Author: SarahSoutoul

redirects/dynamic/docs.jsonc

@@ -49,6 +49,11 @@
     "destination": "/docs/:sdk/reference/hooks/:path*",
     "permanent": true,
   },
+  {
+    "source": "/docs/references/:path/custom-signup-signin-pages",
+    "destination": "/docs/guides/development/custom-sign-in-or-up-page",
+    "permanent": true,
+  },
   {
     "source": "/docs/references/:path*",
     "destination": "/docs/reference/:path*",