Add new Use cases page

Author: pwseg

src/engage/journeys/v2/use-cases.md

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+---
+title: Journeys (V2) Use Cases
+plan: engage-foundations
+---
+
+Event-Triggered Journeys give you the tools to create real-time, personalized flows that react to user behavior as it happens. This page explains sample use cases to help you get started with common patterns like confirmations, abandonment campaigns, and re-engagement flows.
+
+Each example includes a short explanation, key steps to configure, and optional sample event payloads.
+