docs: explain hashlink JS for main navigation

Author: adamjohnson

static/js/page.js

@@ -3,6 +3,7 @@ document.addEventListener('DOMContentLoaded', function() {
         'rh-navigation-primary-item[href^="#"], rh-navigation-primary-item > a[href^="#"]'
     );
 
+        // Close dropdowns after clicking an in-page link on mobile viewports
     for (let hashLink of mainNavHashLinks) {
         hashLink.addEventListener('click', function() {
             hashLink.closest('rh-navigation-primary').close();