fix interactive toc

[abernier]

https://css-tricks.com/table-of-contents-with-intersectionobserver/

• wrap any heading in-between content, into a <div>
  • use a rehype plugin for this: get inspiration from src/components/mdx/Code/rehypeCode.ts
  • maybe also src/components/mdx/Toc/rehypeToc.ts
• uncomment/refactor code inside src/components/mdx/Toc/Toc.tsx, based on the the wrapper divs

https://codepen.io/hakimel/pen/BpKNPg

[abernier]

started #320