docs: update .gitignore to include npm config file

.gitignore

@@ -25,6 +25,9 @@ report.[0-9]_.[0-9]_.[0-9]_.[0-9]_.json
 .env.production.local
 .env.local
 
+# npm config (may contain auth tokens for private registries)
+.npmrc
+
 # caches
 .eslintcache
 .cache