How do I add a toolbox that is downloaded off a separate site and not through the Add-on explorer?

So I'm trying to install the following Bifurcation Analysis Toolbox and I can't seem to figure out how, I know the usual route is to go through the add-on explorer but it's not on there.

[Walter Roberson]

https://dercole.faculty.polimi.it/tds/matcont.html

[Jaider]

Thank you! I've been looking all over the place for something like this