docs: add OpenTelemetry output documentation

[Molter73]

Description

Add docs/otel/ with:

• otel.md: how to build with --features otel or make image-otel, configure the OTLP endpoint, and a full Loki + Grafana example
• loki-config.yaml: minimal single-instance Loki configuration
• grafana-datasource.yaml: auto-provisioned Loki datasource
• grafana-dashboard-provider.yaml: dashboard file provider config
• grafana-fact-dashboard.json: example dashboard with event rate, events by type/file pie charts, and a log stream panel

Assisted-by: claude-opus-4-6 noreply@opencode.ai

Checklist

• Patch has a change log entry OR does not need one.
• Investigated and inspected CI test results
• Updated documentation accordingly

Automated testing

• Added unit tests
• Added integration tests
• Added regression tests

If any of these don't apply, please comment below.

Testing Performed

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch mauro/docs/otel

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (mauro/ci/build-test-otel@faefb58). Learn more about missing BASE report.

Additional details and impacted files

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