docs: freeze gh-pages archive

[andreatgretel]

📋 Summary

Freeze the legacy gh-pages MkDocs site as an archive for Data Designer 0.5.7 and older. After the docs.nvidia.com cutover, post-0.5.7 GitHub Pages URLs should redirect to the Fern-hosted NVIDIA docs instead of serving stale MkDocs output.

🔗 Related Issue

N/A

🔄 Changes

✨ Added

• Added a root 404.html fallback for GitHub Pages misses.
• Added redirect.js to route latest, 0.6.0, 0.5.9, and 0.5.8 links to docs.nvidia.com/nemo/datadesigner.
• Added small redirect shim pages under latest, 0.6.0, 0.5.9, and 0.5.8.

🔧 Changed

• Updated the root index.html to redirect to https://docs.nvidia.com/nemo/datadesigner/.
• Updated versions.json so the legacy MkDocs version selector only lists 0.5.7 and older.

🗑️ Removed

• Removed the full MkDocs archives for 0.6.0, 0.5.9, and 0.5.8.
• Replaced the latest symlink to 0.6.0 with a redirect shim directory.

🔍 Attention Areas

⚠️ Reviewers: Please pay special attention to the following:

• This PR targets gh-pages, not main.
• Keep this PR as a draft until the docs.nvidia.com cutover is live.
• redirect.js maps removed version paths to docs.nvidia.com/nemo/datadesigner/v<version>/<path>. Update it before merge if the Fern version routes change.
• The 0.5.7 and older MkDocs archives are intentionally left in place for Fern's legacy-version links.

🧪 Testing

• jq . versions.json
• node --check redirect.js
• git diff --check
• find 0.5.8 0.5.9 0.6.0 latest -type f | sort
• du -sh 0.5.8 0.5.9 0.6.0 latest
• make test passes (N/A: static gh-pages archive branch)
• Unit tests added/updated (N/A: no Python or app logic changed)
• E2E tests added/updated (N/A: redirect-only static archive update)

✅ Checklist

• Follows commit message conventions
• Commits are signed off (DCO)
• Architecture docs updated (N/A: gh-pages archive-only cleanup)