Their issue tracker might be a good start. Besides you might find some more info on their Matrix space.

https://github.com/LemmyNet/lemmy-ui

The page you’re looking for is here.

I noticed earlier that the link explaining the post sorting methods was broken, similarly to the link you had trouble with. I checked the github someone else posted here, and it looks like there’s already a PR merged fixing the sort link, but since you found that other broken link, I bet there’s more similarly broken ones. Not sure if that’s been reported or fixed yet.