Add docs.md rewrite to Vercel config generator

[mintlify]

Summary

• Added /docs.md rewrite rule to the Vercel JSON generator component
• Updated all language variants (en, zh, fr, es)
• Enables accessing the markdown export endpoint when using custom subpaths

The rewrite maps /{subpath}/docs.md to https://{subdomain}.mintlify.app/docs.md.

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Note

Low Risk
Adds a single rewrite rule in documentation snippets; low blast radius and no security- or data-sensitive logic changes.

Overview
Updates the VercelJsonGenerator snippet (and es/fr/zh variants) to include a new Vercel rewrite mapping /{subpath}/docs.md to https://{subdomain}.mintlify.app/docs.md.

This enables the docs.md markdown export endpoint to work when docs are hosted under a custom subpath, without impacting existing rewrite behavior.

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