Add autohighlight to article table of contents

[rossta]

Uses IntersectionObserver to update TOC link color as article headers enter the top of the viewport. Inspired by https://www.smashingmagazine.com/2021/07/dynamic-header-intersection-observer/.

Also includes:

• some updates to CSS—moving files, adding background transition
• improvement for running the default rake task: Brakeman defaults to using a pager for printed reported that required interaction for the command to exit; that's been removed