add basic back to top link to docs sidenavs, fixes #12589

docs/_includes/nav-main.html

-<header class="navbar navbar-static-top bs-docs-nav" role="banner">
+<header class="navbar navbar-static-top bs-docs-nav" role="banner" id="top">
   <div class="container">
     <div class="navbar-header">
       <button class="navbar-toggle" type="button" data-toggle="collapse" data-target=".bs-navbar-collapse">

docs/_layouts/default.html

@@ -44,6 +44,9 @@
                 {% include nav-migration.html %}
               {% endif %}
             </ul>
+            <a class="back-to-top" href="#top">
+              Back to top
+            </a>
           </div>
         </div>
       </div>

docs/assets/css/docs.css

@@ -560,8 +560,8 @@ body {
   padding-bottom: 10px;
 }
 .bs-sidebar .nav .nav > li > a {
-  padding-top:    2px;
-  padding-bottom: 2px;
+  padding-top:    1px;
+  padding-bottom: 1px;
   padding-left: 30px;
   font-size: 12px;
   font-weight: normal;
@@ -577,6 +577,27 @@ body {
   padding-left: 28px;
 }
 
+/* Back to top (hidden on mobile) */
+.back-to-top {
+  display: none;
+  margin-top: 10px;
+  margin-left: 10px;
+  padding: 4px 10px;
+  font-size: 12px;
+  font-weight: 500;
+  color: #999;
+}
+.back-to-top:hover {
+  text-decoration: none;
+  color: #563d7c;
+}
+
+@media (min-width: 768px) {
+  .back-to-top {
+    display: block;
+  }
+}
+
 /* Show and affix the side nav when space allows it */
 @media (min-width: 992px) {
   .bs-sidebar .nav > .active > ul {