Add scroll to active toc

[IZUMIRU]

If the toc is long, you may not know where you are reading when you read the chapter below.

I think there is a matter of preference regarding UX, so I will leave it to you to decide whether to merge or close:)

• http://localhost:4000/tour/tour-of-scala.html
• http://localhost:4000/scala3/book/introduction.html

https://github.com/scala/docs.scala-lang/assets/11635641/ce8cd5ff-62da-4014-958c-ea47b04be72c

[bishabosha]

I think this is great - I was wondering if you might like to improve the sidebar in "book" views as well - see https://docs.scala-lang.org/scala3/book/packaging-imports.html

[IZUMIRU]

@bishabosha Thanks for the review. I tried to fix it.

[bishabosha]

Thanks @IZUMIRU , I love it :) just asking for others opinions

[IZUMIRU]

@bishabosha @adpi2 Thanks for the review! I'd like to ask you to merge this approved PR. Need to wait for other reviews?

[SethTisue]

Thank you!