docs: add v1.9.x changelog entry (#382)

* docs: add v1.9.x highlights to changelog

* docs: refine v1.9.0 changelog entry

* docs: expand v1.9.0 changelog copy

Author: KMKoushik

apps/docs/changelog.mdx

@@ -3,6 +3,26 @@ title: "Changelog"
 description: "Product updates and announcements from Unsend"
 ---
 
+<Update label="March 2026" description="v1.9.0">
+
+### Campaign Personalization with Contact Books
+
+Campaigns now support contact-book variables for personalization, so you can tailor each message with richer subscriber data.
+
+### Double Opt-In Customization
+
+Double opt-in now supports customizable confirmation flows and copy, making it easier to match your onboarding experience and compliance requirements.
+
+### API and SDK Coverage
+
+Contact book support has been expanded in the SDK, dashboard analytics is now available in both the public API and SDK, and a public endpoint to delete campaigns has been added.
+
+### Webhook Filtering by Domain
+
+Webhooks now support multi-domain filtering, making it easier to route, monitor, and manage events across different projects.
+
+</Update>
+
 <Update label="February 2025" description="v1.8.0">
 
 <Frame>