docs: Platform tutorial (#2105)

This adds a platformer tutorial called Ember Quest. I hope I have done a service, because I am tired, lol. I am sure there will be comments. I just want to say, I did my best. I approached this as someone new to Flame, just like I was about 10 months ago. Are there concepts that can be improved, sure. We can always optimize code, but I didn't want any concepts to be super abstract. I had never coded a game before when I began my journey with Flame this year, so things might be a bit simple for experienced game developers, but for myself, I had never even thought about a game loop or animations, etc.

doc/tutorials/platformer/app/lib/main.dart

@@ -0,0 +1,19 @@
+import 'package:flame/game.dart';
+import 'package:flutter/material.dart';
+
+import 'ember_quest.dart';
+import 'overlays/game_over.dart';
+import 'overlays/main_menu.dart';
+
+void main() {
+  runApp(
+    GameWidget<EmberQuestGame>.controlled(
+      gameFactory: EmberQuestGame.new,
+      overlayBuilderMap: {
+        'MainMenu': (_, game) => MainMenu(game: game),
+        'GameOver': (_, game) => GameOver(game: game),
+      },
+      initialActiveOverlays: const ['MainMenu'],
+    ),
+  );
+}