docs(router): use the public routerPresets export

[cangen185-sudo]

📝 Description

Replace the obsolete Registry.Router.presets.orth reference in the English and Chinese Manhattan-router documentation with the exported routerPresets.orth. Update both examples to show importing routerPresets from @antv/x6 and using it as fallbackRouter.

💡 Motivation and Context

Closes #5092.

X6 v3 exports routerPresets through its public entry point; Registry has no Router static property. This change only corrects documentation and does not add an API or change routing behavior.

🧩 Types of changes

• Docs

🔍 Validation

• Extracted the actual Manhattan example from each locale and type-checked it against the current public source entry point using the repository's TypeScript configuration: 0 diagnostics for each locale.
• vitest run __tests__/registry/router: 126 passed.
• git diff --check: passed.
• Full-suite recheck at this exact PR head: 3032 passed, 0 failed across 202 test files; TypeScript passed. This Windows recheck uses a real Arial font registered as the process-local sans-serif, addressing the baseline test's implicit font-width assumption. No test assertion or system font configuration was changed.
• No documentation-site build or browser rendering is claimed.

Codex assisted with documentation audit and validation. Ready for maintainer review.