TOC: Add toc highlight

[nginx-jack]

A rough implementation of a toc highlight.
Styling only adds bolding to prove the js works.
As you scroll, the current header "section" you're on is highlighted. Visible https://frontdoor-test-docs.nginx.com/previews/nginx-hugo-theme/455/nginx/installing-nginx-open-source/#choosing-between-a-stable-or-a-mainline-version

[github-actions]

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[lamATnginx]

Thanks for working on this and adding this big QoL. My main complaint is with the InstersectionObserver could be tweaked more so it picks up on "active" state in the same position as if I had the heading link.

Right now it is registering the next heading as "active" while I am still reading the current heading:

[nginx-jack]

Thanks for working on this and adding this big QoL. My main complaint is with the InstersectionObserver could be tweaked more so it picks up on "active" state in the same position as if I had the heading link.

Right now it is registering the next heading as "active" while I am still reading the current heading:

Yeah makes sense, I'll move the variables that control this behaviour out of the function so they can easily be tweaked and tested. Something for @danielledeleo to play with after the holidays 👍 .