A back-to-top button that is available by adding the following to the .yml file:
theme:
features:
- navigation.top

However, this needs some tweaking to work for us. Specifically, the default is a light grey button that appears at the top of the page whenever you are scrolling up (it disappears when scrolling down). Ideally it would be at the bottom right of the page and remain there when scrolling up or down. See the one on the new website and try to make it similar for consistency! The default text on the button could also be changed to the icon used on the new website.

After this issue has been resolved, Dyalog/documentation#520 can be done.