docs(i18n): document translation workflow architecture

[hxy91819]

Summary

• Replace the outdated Translate All notes with the current Translate Incremental / Translate Full architecture.
• Add a Mermaid architecture map with color-coded control-plane, finalizer, R2 publish, and live-router paths.
• Document R2 Pages as the content publish path, scoped page/locale deploys, workflow-ref script pinning, weekly reconciliation, and remaining partial-success work.

Validation

• python .github/scripts/i18n/tests/test_i18n_scripts.py
• Mermaid render check for docs/.i18n/translation-workflow.md using mmdc with --no-sandbox
• git diff --check -- docs/.i18n/translation-workflow.md

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