Improve the way docs sites work for mobile

Improve the way the docs sites perform on mobile. Specifically:

1. The top navigation should not be a hamburger button on mobile, rather it should be stacked navigation links.

2. On mobile, the sidebar content should behave like a menu. A hamburger menu should reveal the menu of topics. Once a selection is made, the menu closes and the chosen page is loaded. On desktop, it should be a permanent sidebar.