feat(rootly): add On Event trigger component

[nenadilic84]

Summary

Adds a new On Event trigger for the Rootly integration that fires when incident timeline events (notes, annotations) are created or updated.

• Backend: on_event.go implementing the trigger with webhook signature verification, event type filtering, and optional filters for incident status, severity, and event visibility
• Tests: 16 test cases covering signature validation, event filtering, and all optional filter combinations
• Frontend: TypeScript trigger renderer (on_event.ts) displaying timeline event details, parent incident info, and configured filters
• Docs: Auto-generated documentation with example data

Configuration Options

• Events (required): incident_event.created, incident_event.updated
• Incident Status (optional): Filter by parent incident status
• Severity (optional): Filter by parent incident severity
• Visibility (optional): Filter by event visibility (internal/external)

Design Decisions

• Dropped "event kind" and "event source" filters per maintainer feedback on feat: Add rootly.onEvent component #3042
• Emits both incident_event (full timeline event) and incident (parent incident) at the top level for downstream convenience
• Reuses existing WebhookConfiguration, verifyWebhookSignature, and WebhookPayload from the Rootly integration

Closes #2820

Test plan

• All 16 unit tests pass (go test ./pkg/integrations/rootly/... -run Test__OnEvent)
• Go build succeeds (go build ./...)
• Component docs regenerated and in sync
• TypeScript compiles without errors
• Demo video showing: (1) setting up the integration, (2) a Rootly timeline event triggering a workflow

[nenadilic84]

Demo video coming shortly — recording the integration setup and a Rootly timeline event triggering a workflow.