docs: update /config/app-package-json redirect (angular#37774)

With this change we change the redirect for `/config/app-package-json` from `https://webpack.js.org/configuration/optimization/#optimizationsideeffects` to `https://angular.io/guide/strict-mode#non-local-side-effects-in-applications`

The latter page has more details.

PR Close angular#37774

Author: alan-agius4

aio/firebase.json

@@ -128,7 +128,7 @@
       // The below paths are referenced in users projects generated by the CLI
       {"type": 301, "source": "/config/tsconfig", "destination": "/guide/typescript-configuration"},
       {"type": 301, "source": "/config/solution-tsconfig", "destination": "https://devblogs.microsoft.com/typescript/announcing-typescript-3-9/#solution-style-tsconfig"},
-      {"type": 301, "source": "/config/app-package-json", "destination": "https://webpack.js.org/configuration/optimization/#optimizationsideeffects"}
+      {"type": 301, "source": "/config/app-package-json", "destination": "/guide/strict-mode#non-local-side-effects-in-applications"}
     ],
     "rewrites": [
       {