docs: update integration testing guide with comprehensive smoke tests and fixes

- Add detailed external smoke testing section using official Torrust client tools
- Document proper API authentication and nginx proxy usage
- Add troubleshooting for common issues (firewall, proxy headers, tracker visibility)
- Include performance validation and response time expectations
- Update cleanup procedures and resource management
- Add comprehensive summary of successful test results
- Fix tracker configuration: set private=false for public tracker operation
- Document critical configuration fixes and lessons learned
- Improve working directory context and command execution guidance

Author: josecelano