docs: Update COMPLETE_GUIDE.md to reflect simplified template pattern

- Removed all references to reusable-package-test.yml (deleted workflow)
- Replaced Method 1/Method 2 dichotomy with single template approach
- Updated Quick Start: 10 min -> 15-20 min (realistic time)
- Replaced Workflow Patterns section: removed Pattern 1/Pattern 2
- Rewrote Adding Packages section: comprehensive Redis example
- Added Common Customizations: services, networking, binary downloads
- Updated file locations: 4 workflows -> 3 workflows
- Updated infrastructure description: template-package-test.yml replaces reusable workflow
- All examples now show copy/customize template pattern

Author: pareenaverma