add style

doc/_static/xgboost-theme/navbar.html

@@ -0,0 +1,40 @@
+<div class="navbar navbar-default navbar-fixed-top">
+  <div class="container">
+    <div class="navbar-header">
+      <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#navbar" aria-expanded="false" aria-controls="navbar">
+        <span class="sr-only">Toggle navigation</span>
+        <span class="icon-bar"></span>
+        <span class="icon-bar"></span>
+        <span class="icon-bar"></span>
+      </button>
+    </div>
+    <div id="navbar" class="navbar-collapse collapse">
+      <ul id="navbar" class="navbar navbar-left">
+        <li> <a href="{{url_root}}">XGBoost</a> </li>
+        {% for name in ['Get Started', 'Tutorials', 'How To'] %}
+        <li> <a href="{{url_root}}{{name.lower()|replace(" ", "_")}}/index.html">{{name}}</a> </li>
+        {% endfor %}
+        {% for name in ['Packages'] %}
+        <li class="dropdown">
+          <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="true">{{name}} <span class="caret"></span></a>
+          <ul class="dropdown-menu">
+            <li><a href="{{url_root}}python/index.html">Python</a></li>
+            <li><a href="{{url_root}}R-package/index.html">R</a></li>
+            <li><a href="{{url_root}}jvm/index.html">JVM</a></li>
+            <li><a href="{{url_root}}julia/index.html">Julia</a></li>
+            <li><a href="{{url_root}}cli/index.html">CLI</a></li>
+          </ul>
+        </li>
+        {% endfor %}
+        <li> <a href="{{url_root}}/parameter.html"> Knobs </a> </li>
+        <li> {{searchform('', False)}} </li>
+      </ul>
+      <!--
+      <ul id="navbar" class="navbar navbar-right">
+        <li> <a href="{{url_root}}index.html"><span class="flag-icon flag-icon-us"></span></a> </li>
+        <li> <a href="{{url_root}}/zh/index.html"><span class="flag-icon flag-icon-cn"></span></a> </li>
+      </ul>
+      navbar -->
+    </div>
+  </div>
+</div>