Add a sticky table of contents with active states

[ian-whitestone]

This would be useful for longer articles...

Examples

https://ethereum.org/en/developers/docs/scaling/#scaling-overview

Implementations

• https://css-tricks.com/sticky-table-of-contents-with-scrolling-active-states/
• https://www.bram.us/2020/01/10/smooth-scrolling-sticky-scrollspy-navigation/