Add event time model doc

[stacknil]

Summary

• add docs/event-time-model.md defining event_time, observed_time, window_start/window_end, and artifact_generated_at
• document the OpenTelemetry Timestamp / ObservedTimestamp distinction as the reference model while keeping telemetry-lab scoped as a local lab
• link the new engineering doc from README and docs/README.md
• add a focused docs test for the time-field boundaries

Validation

• pytest tests/test_event_time_model_docs.py tests/test_markdown_links.py --basetemp .pytest-artifacts-event-time-doc
• pytest --basetemp .pytest-artifacts-full-event-time-doc
• git diff --cached --check
• staged diff privacy/secret keyword scan