How to make a website scroll vertically slowly while still keeping control of scrolling

I am making a Ruby on Rails website. I would like to implement a functionality that when a user clicked a button, the website would scroll down slowly. I have looked at similar options but most of what I've found takes control of the scrolling away from the user until it ends. I want the user to still be able to scroll wherever they want to while that automatic scroll is happening. Here is an example: https://www.ukulele-tabs.com/uke-songs/vance-joy/riptide-uke-tab-31868.html

Any ideas?

Thanks!