[Proposal] Update sidebar behavior on mobile device

[L1ghtmann]

The current sidebar implementation works wonderfully on a typical browser, with the various pages and subpages hyperlinked on the side. On mobile, however, this sidebar is placed at the bottom of each respective page. This is functional, but non-ideal for navigating the site. I think it's worth updating our code to hide the page-based sidebar and provide a fixed, JS-based button to present the sidebar on mobile.

I have taken a brief look at implementing this, but the current jekyll setup [1, 2] complicates things a bit. That being said, I am by no means a proficient web dev, so if others want to take a crack at it or have other ideas, feel free!