Update nav links to enable dropdowns

[thclark]

Feature request

Use Case

The legacy site navbar allowed menu items to have children rendered in a dropdown. This is no longer the case.

• [ ] Update menu data structure to enable children
• [ ] Re-add dropdown capability using a down/up caret icon to the right of the rendered item
• [ ] Ensure styling works for both mobile drawer items and desktop navbar items
• [ ] Assume only one level of dropdown, we don't want multiple menus