Add floating/sticky TOC for blog posts

[spcanelon]

Ideas:

• Alison, Hugo Apéro, series, very similar to Mattias Geniar, tutorial
• Alison, Hugo Academic, blog
• Garrick, blog, involves JS+CSS
• Hugo, simple
• Yihui, blog, involves JS, found on Hugo issue #1778

[spcanelon]

Noticed the ToC also shows up beneath "Details" in Talks and Publications

• ~Maybe try editing partial sidebar-scaffold.html instead of post-details.html?~
• [X] edit single-sidebar.html layout inside layouts/blog

[spcanelon]

Styled mostly with Hugo Academic CSS

• [x] Add "scrollspy" feature: https://mdbootstrap.com/docs/standard/navigation/scrollspy/

Primary resources used:

• Adding a sticky table of contents in Hugo to posts | Mattias Geniar
• Smooth Scrolling Sticky ScrollSpy Navigation | Bramus

[spcanelon]

• [ ] ~Modify ToC observer so that the focus doesn't jump to the most 'recent' header to scroll into view~
• [x] Modify ToC observer so that it includes parent element and doesn't appear as jumpy

Ideas/resources:

• Using intersectional observers | Bramus
• Intersection Observer API - Web APIs | MDN
• An Explanation of How the Intersection Observer Watches | CSS-Tricks