[docs] make building of C++ tests section collapsable (#4340)

docs/_static/js/script.js

@@ -6,7 +6,7 @@ $(function() {
     if(window.location.pathname.toLocaleLowerCase().indexOf('installation-guide') != -1) {
         $('<style>.closed, .opened {cursor: pointer;} .closed:before, .opened:before {font-family: FontAwesome; display: inline-block; padding-right: 6px;} .closed:before {content: "\\f078";} .opened:before {content: "\\f077";}</style>').appendTo('body');
         var collapsable = ['#build-threadless-version-not-recommended', '#build-mpi-version', '#build-gpu-version',
-                           '#build-cuda-version-experimental', '#build-hdfs-version', '#build-java-wrapper'];
+                           '#build-cuda-version-experimental', '#build-hdfs-version', '#build-java-wrapper', '#build-c-unit-tests'];
         $.each(collapsable, function(i, val) {
             var header = val + ' > :header:first';
             var content = val + ' :not(:header:first)';