[feat] use instance_name for index page title

searx/templates/simple/index.html

@@ -2,7 +2,7 @@
 {% from 'simple/icons.html' import icon_big %}
 {% block content %}
 <div class="index">
-    <div class="title"><h1>SearXNG</h1></div>
+    <div class="title"><h1>{{instance_name}}</h1></div>
     {% include 'simple/simple_search.html' %}
 </div>
 {% endblock %}