Update instructions for how to use navigationLinks

I was confused by this for ~2 hours. I didn't realize that navigationStyle: 'custom' needed to be enabled to use the navigationLinks and was digging through the code wondering why navigationLinks were not showing.

Author: jonleung

site.config.ts

@@ -49,7 +49,7 @@ export default siteConfig({
   pageUrlOverrides: null,
 
   // whether to use the default notion navigation style or a custom one with links to
-  // important pages
+  // important pages. To use `navigationLinks`, set `navigationStyle` to `custom`.
   navigationStyle: 'default'
   // navigationStyle: 'custom',
   // navigationLinks: [