Component: Sub-navigation

[craigcook]

Many (most?) websites organize related groups of pages into connected sections, and provide a secondary navigation to move through pages within that section. We've implemented a subnav on some pages on mozilla.org but never backported that compontent to Protocol. We could start there, but could also take this opportunity to rethink it.

Example from https://www.mozilla.org/firefox/privacy/

We'll be redesigning the main navbar component in the near future and should make considerations for that as well.