move options back out to their config files, improve documentation

modules/components/boot-loaders/grub.nix

@@ -1,8 +1,12 @@
 { inputs, self, ... }: {
   flake.nixosModules.global = { pkgs, config, lib, ... }: {
-    boot.loader.grub = lib.mkIf (config.components.bootLoader == "grub") {
+    boot.loader.grub = lib.mkIf (config.components.bootLoader.selection == "grub") {
       enable = true;
       useOSProber = true;
+      configurationLimit = config.components.bootLoader.maxGenerations;
+      extraConfig = ''
+        timeout ${config.components.bootLoader.timeout}
+      '';
     };
   };
 }