docs(vercel): add comprehensive Vercel integration docs

[0ski]

Expand documentation for the Vercel integration with detailed usage,
installation, environment variable sync, atomic deployments, and
environment mapping. Replace the previous "coming soon" placeholder with
complete instructions and UI flow for connecting via the Trigger.dev
dashboard or the Vercel Marketplace. Explain required GitHub integration,
how env vars sync in both directions, which vars are excluded, and how
to control sync behavior. Describe atomic deployments (default for
production), how they gate Vercel deployments to ensure task/app
consistency, and note related configuration changes. Add tips and notes
to guide setup and troubleshooting.

This provides users with actionable guidance to connect Vercel, map
environments, and keep app and tasks in sync without custom CI scripts.

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Latest commit: f5735b6

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.