refactor: change to litterate config

Configuration is now held by the `.org` files. All `.nix` files are
tangled from the org-mode files.

modules/desktop/xserver.org

@@ -0,0 +1,50 @@
+#+title: X Server
+#+setupfile: ../headers
+
+* X Server
+What’s left of =services.xserver= on my hosts now that GNOME runs on
+Wayland: picking extra video drivers to load, and my own keyboard
+layout. Here’s the skeleton of the =nixos.xserver= module.
+#+begin_src nix :tangle yes
+{
+  flake.modules.nixos.xserver = {
+    config,
+    lib,
+    ...
+  }:
+    with lib; let
+      cfg = config.mySystem.desktop.xserver;
+    in {
+      <<options>>
+      <<config>>
+    };
+}
+#+end_src
+
+** Declaring the Option
+=videoDrivers= lets a host load extra X11 drivers, such as =amdgpu= on
+the machine with an AMD GPU.
+#+name: options
+#+begin_src nix
+options.mySystem.desktop.xserver.videoDrivers = mkOption {
+  type = types.listOf types.str;
+  default = [];
+  example = ["amdgpu"];
+  description = "Extra X11 video drivers to load";
+};
+#+end_src
+
+** Enabling It
+I use the same Bépo AFNOR keyboard layout everywhere, so it’s baked in
+directly rather than exposed as an option.
+#+name: config
+#+begin_src nix
+config.services.xserver = {
+  enable = true;
+  videoDrivers = cfg.videoDrivers;
+  xkb = {
+    layout = "fr";
+    variant = "bepo_afnor";
+  };
+};
+#+end_src