Add Nav Menu component

### Description

Document a common desktop navigation menu component, used for displaying a list of links when a user hovers or activates using a keyboard. This should demonstrate a case where the parent of the navigation menu is a link and a case where it is not (the top level should either act as a toggle to open the submenu, or should link, but not both). It would be good to document a few scenarios, ensuring that each solution is compatible with mouse, keyboard, and touch controls.

### Use Case

The navigation menu could be used in the header navigation of a site, where there are multiple layers of navigation. This would pair with the hamburger menu component to provide a complete picture of implementing a header navigation pattern.