Add version badge to extensibility.md

Added a badge for version 4.0 to the extensibility documentation.

Author: maartenba

src/content/docs/accesstokenmanagement/advanced/extensibility.md

@@ -4,6 +4,9 @@ description: Learn how to extend and customize Duende.AccessTokenManagement, inc
 sidebar:
   label: Extensibility
   order: 50
+  badge:
+    text: v4.0
+    variant: tip
 ---
 
 There are several extension points where you can customize the behavior of Duende.AccessTokenManagement.