feat: Added new docs and images

docusaurus/docs/Flutter/stream_chat_flutter_core/user_list_core.mdx

@@ -18,3 +18,48 @@ exposes builders to build the UI in situations such as loading, empty data, erro
 
 ### Basic Example
 
+[UserListCore] is a simplified class that allows fetching users while
+exposing UI builders.
+A [UserListController] is used to load and paginate data.
+
+```dart
+class UsersListPage extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      body: UsersListCore(
+        filter: Filter.autoComplete('name', 'search_here'),
+        sort: [SortOption('last_message_at')],
+        pagination: PaginationParams(
+          limit: 20,
+        ),
+        errorBuilder: (err) {
+          return Center(
+            child: Text('An error has occured'),
+          );
+        },
+        emptyBuilder: (context) {
+          return Center(
+            child: Text('Nothing here...'),
+          );
+        },
+        emptyBuilder: (context) {
+          return Center(
+            child: CircularProgressIndicator(),
+          );
+        },
+        listBuilder: (context, list) {
+          return UsersPage(list);
+        }
+      ),
+    );
+  }
+}
+```
+
+[UsersBloc] must be the ancestor of this widget. This is necessary since
+[UserListCore] depends on functionality contained within [UsersBloc].
+
+The parameters [listBuilder], [loadingBuilder], [emptyBuilder] and
+[errorBuilder] must all be supplied and not null.
+