docs: [#28] add comprehensive staging deployment testing documentation

- Add docs/testing/ directory structure for manual testing documentation
- Add manual-staging-deployment-testing.md with 8-phase testing framework
- Add template-session.md for tracking individual test sessions
- Add 2025-01-08-issue-28-phase-4-7-staging.md for current Phase 4.7 testing
- Add staging-deployment-testing-guide.md in guides/ for easy discovery
- Establishes systematic approach for Issue #28 Phase 4.7 staging testing
- Provides reusable framework for future staging deployments
- Includes comprehensive session tracking and result documentation

Author: josecelano