add nested menus capability

assets/icons/chevron-down.svg

@@ -0,0 +1,12 @@
+<svg
+  xmlns="http://www.w3.org/2000/svg"
+  viewBox="0 0 20 20"
+  fill="currentColor"
+  aria-hidden="true"
+>
+  <path
+    fill-rule="evenodd"
+    d="M5.23 7.21a.75.75 0 011.06.02L10 11.168l3.71-3.938a.75.75 0 111.08 1.04l-4.25 4.5a.75.75 0 01-1.08 0l-4.25-4.5a.75.75 0 01.02-1.06z"
+    clip-rule="evenodd"
+  />
+</svg>

assets/js/header.js

@@ -0,0 +1,11 @@
+function header_toggle_nested_menu(element) {
+  if (!element) {
+    throw new Error("Could not find button")
+  }
+  let parent_element = element.parentElement;
+  if(!parent_element) {
+    throw new Error("Could not get parent element from button")
+  }
+  let nested_menu = parent_element.querySelector(".header-nested-menu")
+  nested_menu.classList.toggle('hidden');
+}