Highlight current section of sidebar when scrolling

[HonkingGoose]

What feature would you like to have?

The sidebar I've designed contains a list of headings. When you scroll past the heading, the sidebar highlight should change to show the current heading.

What problem will be solved by having the feature?

Makes it clear where you are, both on the page (with the sticky heading in content area), and on the sidebar (with hightlight).

Do you want to help making the feature?

We need to have a sidebar component first, before we can start work on this issue.

I'm designing the sidebar and the highlight in the sidebar, but @Belco90 needs to make the code changes to make the feature functional. 😄