header-option-simple.html renders menus without nested items
header-option-nested.html renders menus with nested items
header-option.html decides which template to render given the menu
js/header.js exposes a method to toggle visibility of nested menus
Nesting is configured using the parent and identifier properties in menus.en.yml. Example

main:
 - name: Product
    identifier: product
    weight: 1
  - name: Analytics
    pageRef: analytics
    weight: 1
    parent: product
  - name: Engagement
    pageRef: engagement
    weight: 2
    parent: product

secondary:
 - name: Engineering
    identifier: engineering
    weight: 1
  - name: Computers
    pageRef: computers
    weight: 1
    parent: engineering
  - name: Rockets
    pageRef: rockets
    weight: 2
    parent: engineering