docs: complete Phase 3 documentation (Infrastructure & GitOps workflows)

## Documentation Updates

### USAGE.md
- Added Infrastructure Workflows section (3 workflows)
  - terraform/validate.yml - Full parameter docs + 2 examples
  - terraform/plan.yml - Full parameter docs + 3 examples
  - terraform/apply.yml - Full parameter docs + 2 examples
- Added GitOps Workflows section (2 workflows)
  - gitops/update-manifests.yml - Full parameter docs + 3 examples
  - gitops/argocd-sync.yml - Full parameter docs + 3 examples
- Updated Table of Contents (9 sections)

### EXAMPLES.md
- Added Terraform CI/CD section (3 complete examples)
  - Complete Terraform Pipeline (validate → plan → apply)
  - Multi-Environment Terraform (matrix strategy, sequential deploys)
  - Terraform with Security Focus (tfsec, Checkov, Gitleaks)
- Added GitOps Pipeline section (4 complete examples)
  - Complete GitOps Pipeline (Docker → update manifests → ArgoCD sync)
  - GitOps with Pull Request (safer production workflow)
  - Multi-Application GitOps (parallel builds, sequential syncs)
  - Helm Values Update (workflow_dispatch with PR creation)
- Updated Table of Contents (10 sections)

## Phase 3 Complete

Infrastructure and GitOps workflows now have comprehensive documentation:
- ✅ Parameter documentation with all inputs/outputs/secrets
- ✅ 15+ usage examples showing common patterns
- ✅ 7 complete end-to-end workflow examples
- ✅ Multi-environment strategies
- ✅ Security-focused pipelines
- ✅ Integration with Phases 1 & 2 workflows

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: samuelho-dev