docs: enhance regionInclude shortcode documentation by adding a code snippet example

Author: anubhav756

DEVELOPER.md

@@ -489,9 +489,17 @@ The `regionInclude` shortcode reads a file, extracts content between `[START reg
 **Syntax:**
 `{{< regionInclude "path/to/file" "region_name" "language" >}}`
 
-**Example:**
+**Example Markdown:**
 `{{< regionInclude "samples/program.js" "program_setup" "javascript" >}}`
 
+**Example Code Snippet (`samples/program.js`):**
+```javascript
+// [START program_setup]
+import { Toolbox } from '@googleapis/genai-toolbox';
+const toolbox = new Toolbox();
+// [END program_setup]
+```
+
 **Source:** [.hugo/layouts/shortcodes/regionInclude.html](.hugo/layouts/shortcodes/regionInclude.html)
 
 ## Building Toolbox