Responsive sidebar

[lukevmorris]

This PR does a few things:

• Remove "Docs" and "About" links from the header nav
• Consolidate "Docs" and "About" into a single "Docs" page acting as a landing page for the documentation. Includes the about blurb.
• Move "Docs" link to the sidebar. Active when not already on /docs/
• Center the logo in the header
• Move a lot of breakpoints to $medium-screen, which seems to be a more natural breakpoint for the sidebar
• When hitting that breakpoint, show the hamburger and hide the sidebar, move the logo off to the left
• When clicking the hamburger, reveal the sidebar on the left as a full-width fixed scrollable menu. Clicking the hamburger again will hide the sidebar.

Above breakpoint

Below breakpoint

With menu toggled open

Closeup of that sweet sweet box-shadow

[lukevmorris]

🙌