Theme updates for Hugo v146

Hugo has overhauled the template system in v146: https://gohugo.io/templates/new-templatesystem-overview/

When we upgrade Hugo, we'll need to re-arrange some files in the theme and make sure everything we change still works.

Their docs currently state:

We have aimed to maintain as much backward compatibility as possible by mapping “old to new,” but some reported breakages have occurred. We’re working on a full overhaul of the documentation on this topic – until then, this is a one-pager with the most important changes.

Let's consider these changes to not be backwards compatible, and just get our files updated to fit the new system.

Done when

• Hugo is updated to v146+ (current latest stable)
• Theme files are re-organized to fit with the new templating architecture