Fix docs nav clickability in narrow browser window.

This fixes https://github.com/prometheus/docs/issues/60.

layouts/docs.html

@@ -2,7 +2,7 @@
 
 <div class="container">
   <div class="row">
-    <div class="col-md-3">
+    <div class="col-md-3 side-nav-col">
       <ul class="nav navbar-nav side-nav">
         <% @items['/docs/'].children.sort_by { |i| i[:sort_rank] || 0 }.each do |i| %>
           <%= toc(i, @item) %>

static/docs.css

@@ -63,10 +63,6 @@ body {
   white-space: nowrap;
 }
 
-.side-nav .fa {
-  color: #555;
-}
-
 footer {
   font-size: 12px;
   color: #333;
@@ -78,11 +74,19 @@ a.sc-logo img {
 }
 
 /* Docs-related styles. */
+.side-nav-col {
+  z-index: 100;
+}
+
 .side-nav {
   margin-top: 20px;
   padding: 5px 20px 20px 20px;
 }
 
+.side-nav .fa {
+  color: #555;
+}
+
 .side-nav a {
   color: #333;
 }