docs: document story engine and PR2 playable loop

src/content/index.ts

@@ -1,7 +1,7 @@
 import { actionDefs, resourceDefs } from './definitions';
 import { buildContent, type Content } from './schema';
-import { buildStoryContent, type StoryContent } from './storySchema';
 import { storyNodeDefs } from './story';
+import { buildStoryContent, type StoryContent } from './storySchema';
 
 const base = buildContent({ resources: resourceDefs, actions: actionDefs });
 const story = buildStoryContent(storyNodeDefs, base.actionsById, base.resourcesById);