DEV: Replace deprecated firstObject with array indexing in docs-topic.gjs (#222)

Update the `docs-topic.gjs` file to replace the deprecated `firstObject` method with standard array indexing. This change enhances code compatibility with newer Ember versions and maintains functionality by directly accessing the first element of the `posts` array.

Author: megothss

assets/javascripts/discourse/components/docs-topic.gjs

@@ -28,7 +28,7 @@ export default class DocsTopic extends Component {
   model() {
     const post = this.store.createRecord(
       "post",
-      this.topic.post_stream?.posts.firstObject
+      this.topic.post_stream?.posts[0]
     );
 
     if (!post.topic) {