docs: add ZeroLogger documentation and logging best practices

[P09s]

Closes #30

This PR adds comprehensive documentation for the existing ZeroLogger integration in Espresso, including:

• How the logger is initialized and injected across components
• Structured JSON log format with standard fields
• Explanation of log levels (DEBUG, INFO, WARN, ERROR)
• How to view logs in development, staging, and production environments
• Important logging best practices (avoid sensitive data, prefer structured fields, use business events, etc.)
• Practical example of adding new logs in Go code
• Notes about testing/validating log output in feature tests

These additions will significantly improve:

• Developer onboarding experience
• Observability understanding
• Consistency of logging across the codebase
• Long-term maintainability

No code changes — purely documentation improvement.

Thanks for reviewing! ☕