Docs: add GitHub Pages / portfolio CI example (Action only, no vendored rules)

[EshwarCVS]

Summary

Add a short docs example for GitHub Pages / CRA / static portfolio repos that consume leash-secrets via the GitHub Action only.

Motivation

Portfolio and personal sites often:

• Deploy from gh-pages or Actions
• Should not vendor .cursor/rules/leash-secrets.mdc into the site repo
• Need a scan-before-deploy gate

Suggested docs content

1. Minimal secret-scan.yml on push/PR (scan-mode: all, fail-on: critical)
2. Deploy workflow with a needs: secret-scan job
3. Note that Cursor/agent rules should be installed via the upstream installer (or global rules), not copied into every consumer repo
4. Recommended excludes once #exclude-bug is fixed: build/**, node_modules/**, dist/**

Acceptance

A docs page or README section with a copy-paste workflow that a GitHub Pages portfolio can adopt without forking leash-secrets content.