searchbox.html: Use docsearch instead of default

This template overrides the default searchbox.html template shipped with
sphinx hence, allowing usage of custom search like DocSearch.

Closes #463

Author: meetmangukiya

_templates/searchbox.html

@@ -0,0 +1,18 @@
+{%- if pagename != "search" and builder != "singlehtml" %}
+<div id="searchbox" style="display: none" role="search">
+    <h3>{{ _('Quick Search') }}</h3>
+    <div><input type="text" name="q"/></div>
+</div>
+<link rel="stylesheet" href="https://cdn.jsdelivr.net/docsearch.js/2/docsearch.min.css" />
+<script type="text/javascript" src="https://cdn.jsdelivr.net/docsearch.js/2/docsearch.min.js"></script>
+<script type="text/javascript">
+    $('#searchbox').show(0);
+    docsearch({
+        apiKey: '4332f80999581036b1bf34e7aa9a2940',
+        indexName: 'coala',
+        inputSelector: '#searchbox > div > input',
+        debug: false // Set debug to true if you want to inspect the dropdown
+        });
+</script>
+
+{%- endif %}