docs: redirect 4.x API doc URLs to 5.x

[faraz152]

Summary

Fixes #9156 — all /okhttp/4.x/ URLs on the project website return 404 because the 4.x API docs were never published to gh-pages. Pages like /features/caching/ that link to /okhttp/4.x/okhttp/okhttp3/-cache/ are broken.

Changes

Both deploy_website.sh and test_docs.sh now generate a docs/4.x/index.html redirect page during the docs build. The redirect:

• Uses JavaScript window.location.replace() to map /4.x/<path> → /5.x/<path> (preserving sub-paths and hash fragments)
• Falls back to <meta http-equiv="refresh"> for non-JS browsers
• Shows a user-friendly message with a manual link

This works because 5.x is the direct successor to 4.x with the same okhttp3 package structure.

What's on gh-pages today

1.x/  ✅ (cherry-picked in deploy_website.sh)
2.x/  ✅ (cherry-picked in deploy_website.sh)
3.x/  ✅ (cherry-picked in deploy_website.sh)
4.x/  ❌ 404 — no cherry-pick, no generated docs
5.x/  ✅ (generated by Dokka)

After this PR, 4.x/ will contain a redirect page pointing to 5.x/.

Testing

1. Verified curl -s -o /dev/null -w "%{http_code}" https://square.github.io/okhttp/4.x/okhttp/okhttp3/-cache/ returns 404
2. Verified curl -s -o /dev/null -w "%{http_code}" https://square.github.io/okhttp/5.x/okhttp/okhttp3/-cache/ returns 200
3. Confirmed deploy_website.sh and test_docs.sh both produce the redirect in docs/4.x/

[pvgoran]

To me, redirecting 4.x documentation to 5.x seems inappropriate and confusing. OkHttp 5.x is not OkHttp 4.x, so their documentation is not interchangeable.

[faraz152]

Good point — a silent redirect could be confusing since the APIs differ between 4.x and 5.x. Happy to change the approach. Should I instead show a notice page explaining that 4.x API docs aren't published separately, with a link to 5.x as a reference? Or would generating actual 4.x docs from the 4.12.0 tag be preferred?