docs(integration-toolkit): external monitoring events + cross-system trace

[karlrank]

Adds a developer-facing page under Integration Toolkit documenting the new External Monitoring Events capability shipping in erp-integration-api (MR !139).

What it covers

• The topology: an external middleware forwards the inbound event and pushes its own monitoring spans, so the Integration Hub becomes the central monitoring point.
• How the cross-system Event Trace is linked via a shared correlation_id (with a sequence diagram).
• POST /v2/integrations/{id}/monitoring/external-events — the span contract (required fields, server-assigned EXTERNAL_* codes, batch semantics).
• Correlating the forwarded inbound event (per-event vs meta.correlation_id).
• GET /v2/integrations/{id}/monitoring/traces/{correlationId} — reading the assembled cross-system trace.

New file only: docs/integrations/integration-toolkit/external-monitoring-events.md (sidebar position 7). No existing pages changed.

Pairs with backend MR !139; merge after/with that so the endpoints referenced here exist.

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