refactor!: restructure and document configuration

shared/nixos/gaming.nix

@@ -0,0 +1,28 @@
+{ pkgs, ... }:
+
+{
+  # Enable Steam.
+  programs.steam = {
+    enable = true;
+
+    remotePlay.openFirewall = true;
+    localNetworkGameTransfers.openFirewall = true;
+    # ^ Enables so we can transfer games to other computers in the network.
+
+    # Add Proton-GE to 'compatibilitytools.d'.
+    extraCompatPackages = with pkgs; [ proton-ge-bin ];
+  };
+
+  # Enable and configure gamemode.
+  programs.gamemode = {
+    enable = true;
+    enableRenice = true;
+    settings = {
+      gpu = {
+        apply_gpu_optimisations = "accept-responsibility";
+        gpu_device = 1;
+        amd_performance_level = "auto";
+      };
+    };
+  };
+}