How to make an index on the left, like on the guide?

I am not sure how this exactly is called - how could I achieve something like this in write.as, so you could jump directly to a heading?

Sry if this is too obvious, I couldn’t find anything regarding to this

I think Navigation bar is the more fitting name for that guide on the side.
Added for any other who might search for this.

I think this might work for you.