Update contributing guide for the bilingual site and new checks

Text shown to users goes in both message files instead of being
Norwegian only. Document lint and CI, and make npm run format cover
the same files as the CI check.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

package.json

@@ -9,7 +9,7 @@
     "build": "next build",
     "start": "next start -p 9000",
     "lint": "eslint .",
-    "format": "prettier --write \"**/*.{js,jsx,ts,tsx,json,md}\""
+    "format": "prettier --write ."
   },
   "dependencies": {
     "@portabletext/react": "^8.0.1",