add warning at the top of the migrating to v4 page

Author: isshaddad

docs/migrating-from-v3.mdx

@@ -6,6 +6,20 @@ description: "What's new in v4, how to migrate, and breaking changes."
 import NodeVersions from "/snippets/node-versions.mdx";
 import MigrateV4UsingAi from "/snippets/migrate-v4-using-ai.mdx";
 
+<Warning>
+  **Action required: Trigger.dev v3 deprecation**
+
+We're retiring Trigger.dev v3. **New v3 deploys will stop working from 1 April 2026.** Trigger.dev v4 is stable, fully supported, and recommended for all users.
+
+**Key dates:**
+
+- **1 April 2026** — New v3 deploys will no longer work. Existing v3 runs will continue to execute.
+- **1 July 2026** — v3 will be fully shut down. All v3 runs will stop executing.
+
+**What you need to do:** Migrate to v4 before April to avoid disruption to your task executions. The migration takes about 2 minutes — follow the steps on this page below. If you have questions or need help, [contact us](https://trigger.dev/contact) or reach out in our [Discord](https://trigger.dev/discord).
+
+</Warning>
+
 ## What's new in v4?
 
 | Feature                                                              | Description                                                                                                                                                                                                |