Components: ExternalLink, add story (#18084)

This update adds a story for the ExternalLink component.
Storybook knobs were added to better demonstrate the component's
properties.

packages/components/src/external-link/stories/index.js

@@ -0,0 +1,17 @@
+/**
+ * External dependencies
+ */
+import { text } from '@storybook/addon-knobs';
+/**
+ * Internal dependencies
+ */
+import ExternalLink from '../';
+
+export default { title: 'ExternalLink', component: ExternalLink };
+
+export const _default = () => {
+	const title = text( 'children', 'WordPress' );
+	const href = text( 'href', 'https://wordpress.org' );
+
+	return <ExternalLink href={ href }>{ title }</ExternalLink>;
+};