Nav should not show in half-screen sized viewports, or should be hideable by user

[jamesliudotcc]

A common usage pattern for docs sites is to split a screen between the docs and the code. Or a terminal. On the Fly docs, the nav shows, making it harder to read the docs.

The nav bar does hide when the viewport is narrow enough.

One easy visual pattern is to have a button that hides the nav bar.