Add a section to for a NixOS graphical configuration in the VM tutorial

[fricklerhandwerk]

Problem

An obvious next step after getting into the subject with a lightweight VM is to prototype a graphical setup before getting into bare-metal installation. Apparently it's entirely non-obvious how to actually do that, given you need to "somehow know" the three (!) configuration options that have to be set to make it work, compounded by the existence of many display managers and window managers, and the choice between X and Wayland.

Proposal

Add a section at the end of the tutorial that shows a working graphical VM configuration, including how to invoke it, and point to search.nixos.org/options for more... options.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/prototyping-nixos-configuration-on-ubuntu-question-about-graphics/40348/5