doc/search: do not search with less than 3 chars

Please consider this a quickfix to avoid mass results while typing in
the global search field. We need farther improvements here:
* compact mode: e.g. title/first match on one row
* show a hint when less than 3 chars have been provided
* allow two chars once we fixed the above

modules/doc/application/controllers/SearchController.php

@@ -25,6 +25,10 @@ class Doc_SearchController extends DocController
             )
         );
         $search->setUrl('doc/icingaweb/chapter');
+        if (strlen($this->params->get('q')) < 3) {
+            $this->view->searches = array();
+            return;
+        }
         $searches = array(
             'Icinga Web 2' => $search
         );