feat: Make game sub

- Move game and intro to own collection
- Add index
- Add brand placeholder to shortcodes
- Add variant of placeholder to menu

source/screens/demo/examples/game/index.liquid

@@ -0,0 +1,24 @@
+---
+title: Game
+tags:
+- demoExample
+- index
+---
+{% layout 'hippie/simple.liquid' %}
+
+{% block title %}{{ title }}{% endblock %}
+
+{% block body %}
+<div class="sec_main_center">
+	<nav role="doc-toc">
+		<h1>{{ title }}</h1>
+		<ul class="link">
+			{%- for game in collections.game -%}
+				<li>
+					<a href="{{ game.page.url }}">{{ game.data.title }}</a>
+				</li>
+			{%- endfor -%}
+		</ul>
+	</nav>
+</div>
+{% endblock %}