fix: add client-side redirect for moved subscribe-data page

[harsh62]

Summary

• Adds a client-side redirect page at the old /build-a-backend/data/subscribe-data/ URL that redirects to the new /frontend/data/subscribe-data/ location
• The page was moved during content restructuring (feat: docs content restructuring for frontend and backend.  #8531) and a redirect exists in redirects.json, but users are reporting 404s on production — suggesting the Amplify Hosting redirect isn't being applied

Why the redirect wasn't working

The redirects.json file contains the correct redirect rule, but it appears Amplify Hosting isn't picking it up. This could be because:

1. The redirect rules need to be manually synced to the Amplify Console
2. The <platform> wildcard syntax may not be matching as expected
3. There may be a deployment lag

Broader concern

There are 31 redirect rules in redirects.json for pages moved from /build-a-backend/ to /frontend/ during content restructuring. If the hosting redirects aren't working, ALL of these could be 404s. This PR fixes the reported URL, but we should investigate the redirect deployment pipeline to fix them all.

Test plan

• Visit /react/build-a-backend/data/subscribe-data/ — should redirect to /react/frontend/data/subscribe-data/
• The actual content page at /react/frontend/data/subscribe-data/ loads correctly
• Build passes

[harsh62]

Closing this PR — the hacky MDX redirect page is not the right approach.

Root cause

The redirects.json file has the correct redirect rules, but none of them are working on production. I tested multiple URLs:

• /react/build-a-backend/data/subscribe-data/ → 404 (should redirect to /react/frontend/data/subscribe-data/)
• /react/build-a-backend/data/query-data/ → 404 (should redirect to /react/frontend/data/query-data/)
• /react/build-a-backend/auth/connect-your-frontend/sign-in/ → 404 (should redirect to /react/frontend/auth/sign-in/)

All 31 redirect rules from the content restructuring PR (#8531) are broken. The redirects.json file is in the repo but Amplify Hosting isn't applying them.

Required fix

The redirect rules need to be uploaded to the Amplify Hosting Console under "Rewrites and redirects". The redirects.json file in the repo is not automatically consumed by Amplify Hosting — it needs to be manually synced or there needs to be a deployment script that pushes these rules to the Amplify API.