make restored top navbar collapsable

docs/_includes/nav-main.html

 <div class="navbar navbar-fixed-top bs-docs-nav">
   <div class="container">
     <a href="/" class="navbar-brand">Bootstrap</a>
-    <div class="nav-collapse collapse">
+    <button class="navbar-toggle" type="button" data-toggle="collapse" data-target=".bs-navbar-collapse">
+      <span class="icon-bar"></span>
+      <span class="icon-bar"></span>
+      <span class="icon-bar"></span>
+    </button>
+    <div class="nav-collapse collapse bs-navbar-collapse">
       <ul class="nav navbar-nav">
         <li{% if page.slug == "welcome" %} class="active"{% endif %}>
           <a href="/welcome">Overview</a>