Update main.md release to indicate may be out of date on a branch (#24810)

docs/en/releases/main.md

@@ -2,6 +2,17 @@
 
 <Badge type="danger" text="Alpha" />
 
+<script setup>
+import { useData } from 'vitepress'
+const { site } = useData();
+</script>
+
+<div v-if="site.title !== 'PX4 Guide (main)'">
+  <div class="custom-block danger">
+    <p class="custom-block-title">This page is on a release bramch, and hence probably out of date. <a href="https://docs.px4.io/main/en/releases/main.html">See the latest version</a>.</p>
+  </div>
+</div>
+
 This contains changes to PX4 `main` branch since the last major release ([PX v1.15](../releases/1.15.md)).
 
 ::: warning