fix: add Vercel deploy config and workflow for docs site

[MeGaurav4]

Description

The Vercel docs site at headroom-docs.vercel.app had no automated deployment pipeline, so newly added pages (persistent-installs, savings) return 404 despite existing in the repo and building correctly locally.

Closes #1730

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking function added)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Performance improvement
• Code refactoring (no functional changes)

Changes Made

• Add docs/vercel.json with explicit Next.js project config (framework, build/install commands)
• Add deploy-vercel job to .github/workflows/docs.yml to auto-deploy on pushes to main touching docs/**

Testing

• Unit tests pass (pytest)
• Linting passes (ruff check .)
• Type checking passes (mypy headroom)
• New tests added for new functionality
• Manual testing performed

Test Output

Local build verification:
cd docs && npm ci && npm run build
Build succeeded - persistent-installs and savings pages

Real Behavior Proof

• Environment: Linux x86_64, Node.js 20
• Observed result: Both pages build and render correctly locally
• Not tested: Live Vercel deployment (requires maintainer secrets)

Review Readiness

• I have performed a self-review
• This PR is ready for human review

Additional Notes

Requires three repo secrets: VERCEL_TOKEN, VERCEL_ORG_ID, VERCEL_PROJECT_ID.

[github-actions]

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