Taxonomies[0] are the way to go if you'd like to group content on your site by something like series (although you could "just" use tags, which Hugo enables out of the box). I use "series" as a taxonomy, for which an initial bit of code is here[1]. Given that, depending on how comfortable you are with Hugo templating, you could probably use `.Site.Taxonomies.series` to iterate over all the "series".
[0] https://gohugo.io/content-management/taxonomies/
[1] https://dev.danilafe.com/Web-Projects/blog-static/src/commit...