[Exeloo]

What does this PR do?

No linked issue. This PR adds redirects for the engine pages that move out of the site in NanoForge-dev/Engine (docs/rewrite-content): /docs/engine/lib-graphic-3d/* and /docs/engine/graphics-3d/editor-proposal now go to /docs/engine/graphics-3d/index. Without them, those URLs 404 once the next engine release syncs its docs.

How do you test this PR?

• Before: the moved pages would 404 after the next engine sync.
• After: the redirects are in docs.json.
• docs-sync check, then mint validate, mint broken-links and mint a11y on a scratch copy of the site with all four source branches (engine, cli, loader, editor) and the docs redirects synced in: all pass, 83 pages.
• pnpm exec prettier --check and the pre-push hook (lint, tests): pass.

Merge danger

Door: two-way, docs.json only.
Blast radius: the live docs.nanoforge.eu, at merge. Merging before the engine release is fine. If Mintlify gives the redirect precedence over a page that still exists, those research pages simply stop being reachable a little early.

Quality gate

Lane	Result
Docs	the change is docs
Correctness	independent reviewer: findings fixed in the last commit
Accessibility (content)	pass (alt text, headings, link text; mint a11y)
Security (content)	pass, no secrets or internal URLs
Style	style-reviewer: pass
Performance	N/A (docs)