build-index: generate top-level copy of the index

package.json

@@ -13,7 +13,7 @@
     "lint-markdown": "markdownlint pages/**/*.md",
     "lint-tldr": "tldr-lint ./pages",
     "test": "bash -c 'markdownlint pages/ && tldr-lint ./pages 2>&1 | tee test_result; test ${PIPESTATUS[0]} -eq 0'",
-    "build-index": "node ./scripts/build-index.js > pages/index.json"
+    "build-index": "node ./scripts/build-index.js | tee pages/index.json > index.json"
   },
   "repository": "tldr-pages/tldr",
   "author": "Romain Prieto",