docs: [#14] update integration testing guide with network architecture and proper testing procedures

- Add critical network architecture understanding (double virtualization)
- Document proper API testing procedures with authentication tokens
- Clarify port access rules (nginx proxy vs internal Docker ports)
- Add comprehensive endpoint testing examples with jq
- Improve manual cleanup instructions for libvirt volumes
- Document common testing mistakes and correct procedures
- Add troubleshooting for volume conflicts and deployment issues
- Include advanced testing commands and monitoring verification
- Update timing expectations and success indicators

Author: josecelano