Ref Guide: Redesign HTML version landing page

We've had the same first page of the Ref Guide for a long time, and it's probably fine as far as it goes, but that isn't very far. It's effectively a wall of text.

Since we've got the ability to work with an online presentation, and we have some tools available already in use (BootstrapJS, etc.), we can do some new things.

I've got a couple ideas I was playing with a few months ago. I'll dust those off and attach some screenshots here + a patch or two. These will, of course, work for the PDF so I'll include something to show that too (it can also be snazzier).