docs: fix dead external links

[ZaynJarvis]

Summary

• Replace docs-site relative links that point outside the VitePress docs root with GitHub source links
• Fix EN/ZH multi-tenant, metrics, observability, deployment, and Vikingbot reference links
• Replace a missing encryption design-doc link with the existing multi-tenant docs, and fix design-note source links

Verification

• Ran a local Markdown link sweep over docs/; no rendered relative links resolve outside docs/ or to missing files
• npm run docs:build

[github-actions]

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