Docs: Add missing CI/CD secret requirements for Vercel workflows

[vjalumuru]

Problem

The repository workflows require Vercel secrets, but current docs do not clearly document them. This makes CI/CD setup error-prone for new maintainers and forks.

Missing documentation

Please document required GitHub Actions secrets for deployment workflows:

• VERCEL_TOKEN
• VERCEL_ORG_ID
• VERCEL_PROJECT_ID

Where this is used

• .github/workflows/vercel_deploy.yaml
• .github/workflows/deploy_preview.yml
• .github/workflows/remove_deploy_preview.yml

Suggested fix

Add a CI/CD prerequisites section in README.md (or a dedicated docs page) that includes:

1. Required secrets and what each is used for.
2. Which workflows depend on each secret.
3. A short setup guide for adding secrets in GitHub repo settings.
4. Expected behavior when secrets are missing (workflow failures).

Acceptance criteria

• Docs explicitly list all required Vercel secrets.
• Docs map secrets to workflows.
• New maintainer can configure CI/CD without reading workflow YAML files.
• README links to the CI/CD docs section.