Consuming upstream themes in Hugo

This involves adding the upstream theme as a git submodule in the repository.

I don't work with git submodules on a regular basis so it'd an enormous context switch to maintain the linking of the theme.

This involves just downloading the theme into the themes/ folder. I've thoughtfully crafted this operation using make targets.

If I had known about Hugo Submodules, I would've went that approach but I've invested so much effort to finally achieve a perfect Makefile that I'm sticking to it.

The Makefile has so many intricacies to achieve reproducibility that I'm now using it as a reference for showing how it's done.

This involves configuring the theme field with github.com/nanxiaobei/hugo-paper and hugo will fetch the theme from the internet when compiling the website.