feat(devx): introduce storybook

First shot at introducing storybook, with stories for the keycloak
theme pages already in the repo.

src/keycloak-theme/pages/MyExtraPage2.stories.tsx

@@ -0,0 +1,29 @@
+import { ComponentMeta } from '@storybook/react';
+import KcApp from '../KcApp';
+
+import { useKcStoryData } from '../../../.storybook/data'
+
+export default {
+  title: 'My Extra Page 2',
+  component: KcApp,
+  parameters: {
+    layout: 'fullscreen',
+  },
+} as ComponentMeta<typeof KcApp>;
+
+const pageId = 'my-extra-page-2.ftl'
+
+export const Default = () => {
+  const { kcContext } = useKcStoryData({ pageId })
+  return <KcApp kcContext={kcContext} />
+}
+
+export const InFrench = () => {
+  const { kcContext } = useKcStoryData({ pageId, locale: { currentLanguageTag: 'fr' } })
+  return <KcApp kcContext={kcContext} />
+}
+
+export const WithCustomValue = () => {
+  const { kcContext } = useKcStoryData({ pageId, someCustomValue: 'Foo Bar Baz' })
+  return <KcApp kcContext={kcContext} />
+}