Add instructions to upgrade from mint.json to global settings file (#507)

Author: ricardonunez-io

settings/global.mdx

@@ -6,6 +6,21 @@ icon: "wrench"
 
 Every documentation site requires a `docs.json` file that contains the core configuration settings. This file controls everything from styling and navigation to integrations and analytics.
 
+<Note>
+  If you're currently using the legacy `mint.json` configuration file, please update the Mintlify CLI:
+
+  ```sh
+  npm i -g mintlify@latest
+  ```
+
+  And run the new `upgrade` command in your docs repository:
+  ```sh
+  mintlify upgrade
+  ```
+
+  You should now be using the new `docs.json` configuration file. Feel free to delete the `mint.json` file from your repository.
+</Note>
+
 ## Properties
 
 ### Customization