Move 3.x docs notice to partial.

docs/_includes/notice-previous.html

@@ -0,0 +1,7 @@
+<section class="notice-previous">
+  <div class="container text-center">
+    <a href="http://select2.github.io/select2/">Looking for the Select2 3.5.2 docs?</a>
+    We have moved them to a new location
+    <a href="announcements-4.0.html">while we push forward with Select2 4.0</a>.
+  </div>
+</section>

docs/index.html

@@ -22,13 +22,7 @@ slug: home
   </div>
 </main>
 
-<section class="notice-previous">
-  <div class="container text-center">
-    <a href="http://select2.github.io/select2/">Looking for the Select2 3.5.2 docs?</a>
-    We have moved them to a new location
-    <a href="announcements-4.0.html">while we push forward with Select2 4.0</a>.
-  </div>
-</section>
+{% include notice-previous.html %}
 
 <div class="container">
   <section id="lead" class="lead">