Add announcment bar for 1.6 (#1485)

Author: cwickham

_quarto.yml

@@ -21,6 +21,10 @@ website:
   repo-actions: [edit, issue]
   page-navigation: true
   bread-crumbs: true
+  announcement: 
+     content: "🎉 Quarto 1.6 released! [Download](/docs/download/) | [Read More](/docs/blog/posts/2024-11-25-1.6-release)"
+     position: below-navbar
+     dismissable: true
   search:
     show-item-context: true
     type: overlay