Add default settings file support for Dank Material Shell (DMS)

- Introduced a new option `defaultSettingsFile` in the DMS configuration to allow host-specific first-run settings.
- Updated the home-manager module to read from the specified settings file if provided, otherwise falling back to the bundled defaults.
- Enhanced the documentation in the default settings file to clarify its purpose and usage.

modules/hosts/14900k/configuration.nix

@@ -70,6 +70,7 @@
             ".#14900k"
           ];
           enableRbwLockToggle = true;
+          defaultSettingsFile = ./_private/dms-default-settings.json;
         };
       };