Restructure `computer` and `server` folders.

[soupglasses]

Is your feature request related to a problem? Please describe.

Currently a lot of folders inside ./nixos follows a set of "concepts" so to say, like hardware, mixins, modules, and roles. It seems out of place that the default configs for servers and computers are put into this top level, especially since they are both essentially single files of default.nix.

Describe the solution you'd like

This is very much up in the air, and very much for discussion. But something among the lines of ./nixos/kind/server.nix or ./nixos/kinds/computer/default.nix could be a better structure here to follow the idea of having top level "concepts".

Describe alternatives you've considered

Continuing having the ./nixos/computer and ./nixos/server folders.

Additional context