docs: restructure documentation

docs/manual/installation/standalone/nixos.md

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+# Standalone Installation (NixOS) {#ch-standalone-nixos}
+
+The following is an example of a barebones vim configuration with the default theme enabled.
+
+```nix
+{
+  inputs.neovim-flake = {
+    url = "github:notashelf/neovim-flake";
+    inputs.nixpkgs.follows = "nixpkgs";
+  };
+
+  outputs = {nixpkgs, neovim-flake, ...}: let
+    system = "x86_64-linux";
+    pkgs = nixpkgs.legacyPackages.${system};
+    configModule = {
+      # Add any custom options (and feel free to upstream them!)
+      # options = ...
+
+      config.vim = {
+        theme.enable = true;
+      };
+    };
+
+    customNeovim = neovim-flake.lib.neovimConfiguration {
+      modules = [configModule];
+      inherit pkgs;
+    };
+  in {
+    # this will make the package available as a flake input
+    packages.${system}.neovim = customNeovim.neovim;
+
+    # this is an example nixosConfiguration using the built neovim package
+    nixosConfigurations = {
+      yourHostName = nixpkgs.lib.nixosSystem {
+        # ...
+        modules = [
+          ./configuration.nix # or whatever your configuration is
+
+          # this will make wrapped neovim available in your system packages
+          {environment.systemPackages = [customNeovim.neovim];}
+        ];
+        # ...
+      };
+    };
+  };
+}
+```
+
+Your built neovim configuration can be exposed as a flake output, or be added to your system packages to make
+it available across your system. You may also consider passing the flake output to home-manager to make it available
+to a specific user.