document github link option

Author: devalog

fern/products/docs/pages/customization/what-is-docs-yml.mdx

@@ -242,14 +242,20 @@ navbar-links:
     text: Login
     href: https://example.com/login
     rounded: false
+  - type: github
+    value: https://github.com/example-company/fern
 ```
 
 <ParamField path="type" type="enum" required={false}>
-  One of `outlined`, `minimal`, or `filled`. This value controls the styling of the button. 
+  One of `outlined`, `minimal`, `filled`, or `github`. This value controls the styling of the button. 
 </ParamField>
 
 <ParamField path="href" type="string" required={false}>
-  The url once you click on the button. Example: https://buildwithfern.com/contact
+  The URL once you click on the button. Example: https://buildwithfern.com/contact
+</ParamField>
+
+<ParamField path="value" type="string" required={false}>
+  The URL to a GitHub repository. Similar to `href`, but specifically for GitHub repository links. This field is used when `type` is set to `github`. Example: https://github.com/example-company/fern
 </ParamField>
 
 <ParamField path="text" type="string" required={false}>