Add temp go link for json_codable (#5780)

Author: MaryaBelanger

firebase.json

@@ -151,6 +151,7 @@
       { "source": "/go/analysis-server-protocol", "destination": "https://htmlpreview.github.io/?https://github.com/dart-lang/sdk/blob/main/pkg/analysis_server/doc/api.html", "type": 301 },
       { "source": "/go/automated-publishing", "destination": "/tools/pub/automated-publishing", "type": 301 },
       { "source": "/go/cloud", "destination": "/server/google-cloud?utm_source=go-link&utm_medium=referral&utm_campaign=go-cloud", "type": 301 },
+      { "source": "/go/json-codable", "destination": "https://github.com/dart-lang/sdk/tree/main/pkg/json", "type": 301 },
       { "source": "/go/content-hashes", "destination": "/tools/pub/glossary#content-hashes", "type": 301 },
       { "source": "/go/core-lints", "destination": "https://github.com/dart-lang/lints", "type": 301 },
       { "source": "/go/dart-fix", "destination": "/tools/dart-fix", "type": 301 },