docs: update navigation groups documentation to include link for expanded field

Author: skeptrunedev

changelog.mdx

@@ -32,7 +32,7 @@ noindex: true
 ## API playground and navigation
 
 - Multiple API playground response codes now display in a controlled styled select menu instead of the system default select menu when focused
-- You can now use the `expanded` fields on navigation groups in your `docs.json` to make them be default open
+- You can now use the [`expanded` field on navigation groups in your `docs.json` to make them be default open](https://mintlify.com/docs/navigation#default-expanded-state)
 
 ## SEO and UI