Don't require setConfig and fix docs

docs/app/app.ts

@@ -12,7 +12,7 @@ if (macroCondition(isDevelopingApp())) {
 }
 
 setConfig({
-  rootElement: 'body',
+  rootElement: config.APP['rootElement'] as string | undefined,
 });
 
 export default class App extends Application {

docs/app/components/snippets/installation-2.js.txt

@@ -3,11 +3,11 @@ import { setConfig } from 'ember-basic-dropdown/config';
 
 // Basic usage:
 setConfig({
-  rootElement: config.APP.rootElement, // Default is 'body' (or '#ember-testing' in tests)
+  rootElement: config.APP.rootElement, // config.APP.rootElement is by default 'undefined' and '#ember-testing' in tests
 });
 
 // Advanced: If you need to override globally the destination element ID (BasicDropdownWormhole), you can do:
 setConfig({
   destination: 'my-custom-destination-id',
-  rootElement: config.APP.rootElement, // Default is 'body' (or '#ember-testing' in tests)
+  rootElement: config.APP.rootElement, // config.APP.rootElement is by default 'undefined' and '#ember-testing' in tests
 });

src/config.ts

@@ -1,31 +1,14 @@
 export interface Config {
   destination?: string;
-  rootElement: string;
+  rootElement?: string;
 }
 
-let _config: Config = {
-  rootElement: '',
-};
-
-let configSet = false;
+let _config: Config = {};
 
 export function setConfig(config: Config) {
-  if (!config.rootElement) {
-    throw new Error(
-      "ember-basic-dropdown: 'rootElement' is required in the config. See installation instructions for more details.",
-    );
-  }
-
   _config = config;
-  configSet = true;
 }
 
 export function getConfig(): Config {
-  if (!configSet) {
-    throw new Error(
-      'ember-basic-dropdown: setConfig was not called before accessing config. See installation instructions for more details.',
-    );
-  }
-
   return _config;
 }