refactor: Move nix outside of .config dir

Technically not a config but a compose file, so I think it's better to
have outside of `.config` dir

nix/README.md

@@ -0,0 +1,97 @@
+# `null2264`'s Nix Setup
+
+You need to install Nix before following this guide: `sh <(curl -L https://nixos.org/nix/install)` and symlink this dir to `~/.config/nix`
+
+## Structure
+
+```
+├── flake.lock
+├── flake.nix
+├── lib                 # Self-explanatory, it contains helper functions
+│   ├── mkCommon.nix
+│   └── mkSystem.nix
+├── modules             # Contains shared configuration across system, across user
+│   └── home-manager
+│       └── floorp.nix
+├── systems             # Contains configurations depending on what type OS nix is being used in
+│   ├── darwin          # macOS
+│   │   ├── configuration.nix
+│   │   └── default.nix
+│   └── nix             # Linux-alike
+│       ├── default.nix
+│       └── potato
+│           └── default.nix
+└── users               # Contains configurations for multi-user setup
+    ├── default.nix
+    └── ziro
+        ├── darwin.nix
+        ├── default.nix
+        └── linux.nix
+```
+
+## Setup
+
+### macOS
+
+> [!NOTE]
+> If you're using Alfred, to make Applications installed via Nix to appear on Alfred:
+> - Open Alfred Preferences
+> - General > Default Results > Extras > Click `Advanced`
+> - Add `com.apple.alias-file`
+
+Nix in macOS is handled by [nix-darwin](https://github.com/LnL7/nix-darwin).
+
+#### Initial
+
+This is done because nix-darwin commands is not yet added to PATH, should be a one-time thing
+
+```zsh
+nix-env -iA nixpkgs.git
+# Run `sudo chown $USER /nix/var/nix/profiles/per-user/$USER` if that returns error
+
+nix build .#darwinConfigurations.<host>.system
+
+./result/sw/bin/darwin-rebuild switch --flake .#<host>
+# or
+nix run nix-darwin -- switch --flake .#<host>
+```
+
+#### Rebuild
+
+After initial setup, you should now be able to use the command directly:
+
+```sh
+darwin-rebuild build --flake .  # or you can specify the hostname with `--flake . #<hostname>`
+
+# or if you're feeling lucky
+
+darwin-rebuild switch --flake .
+```
+
+### Other
+
+Nix in non-NixOS Linux is handled by [system-manager](https://github.com/numtide/system-manager).
+
+#### Rebuild
+
+Unfortunately, root access is required in order to use system-manager.
+
+```sh
+sudo -i nix run 'github:numtide/system-manager' --extra-experimental-features "nix-command flakes" -- switch --flake $PWD
+```
+
+### Home
+
+This is for per-user setup. Instead of managing the entire system, you're
+managing one user at a time. This is handled by
+[home-manager](https://github.com/nix-community/home-manager)
+
+#### Rebuild
+
+```sh
+home-manager build --flake .  # or you can specify the user with `--flake . #<username>@<hostname>`
+
+# or if you're feeling lucky
+
+home-manager switch --flake .
+```