docs: Add a section on Nix itself

tools/nixery/docs/src/nix.md

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+# Nix
+
+These sections are designed to give some background information on what Nix is.
+If you've never heard of Nix before looking at Nixery, this might just be the
+page for you!
+
+[Nix][] is a functional package-manager that comes with a number of advantages
+over traditional package managers, such as side-by-side installs of different
+package versions, atomic updates, easy customisability, simple binary caching
+and much more. Feel free to explore the [Nix website][Nix] for an overview of
+Nix itself.
+
+Nix uses a custom programming language also called Nix, which is explained here
+[on its own page][nix-1p].
+
+In addition to the package manager and language, the Nix project also maintains
+[NixOS][] - a Linux distribution built entirely on Nix. On NixOS, users can
+declaratively describe the *entire* configuration of their system and perform
+updates/rollbacks to other system configurations with ease.
+
+Most Nix packages are tracked in the [Nix package set][nixpkgs], usually simply
+referred to as `nixpkgs`. It contains tens of thousands of packages already!
+
+Nixery (which you are looking at!) provides an easy & simple way to get started
+with Nix, in fact you don't even need to know that you're using Nix to make use
+of Nixery.
+
+[Nix]: https://nixos.org/nix/
+[nix-1p]: nix-1p.html
+[NixOS]: https://nixos.org/
+[nixpkgs]: https://github.com/nixos/nixpkgs