Update docs

Author: demchenkoalex

doc/firebase-overview.md

@@ -1,6 +1,7 @@
 ---
 id: firebase-overview
 title: Overview
+slug: /
 ---
 
 Flyer Chat is a platform for creating in-app chat experiences using Flutter or React Native. This is the documentation for Firebase BaaS implementation for Flutter.

doc/firebase-usage.md

@@ -11,10 +11,10 @@ import 'package:flutter_firebase_chat_core/flutter_firebase_chat_core.dart';
 
 await FirebaseChatCore.instance.createUserInFirestore(
   types.User(
-    firstName: 'Alex',
+    firstName: 'John',
     id: credential.user!.uid, // UID from Firebase Authentication
     imageUrl: 'https://i.pravatar.cc/300',
-    lastName: 'Demchenko',
+    lastName: 'Doe',
   ),
 );
 ```