add basic back to top link via affix

docs/assets/css/docs.css

@@ -28,6 +28,26 @@ body {
   padding-right: 15px;
 }
 
+/* Back to top link */
+.bs-docs-top {
+  float: left;
+  padding: 7px 15px;
+  font-weight: 500;
+  color: #999;
+  background-color: #eee;
+  border-radius: 4px;
+}
+.bs-docs-top:hover {
+  color: #fff;
+  text-decoration: none;
+  background-color: #999;
+}
+.bs-docs-top.affix {
+  position: fixed;
+  right:  15px;
+  bottom: 15px;
+}
+
 
 
 /* Sections

docs/assets/js/application.js

@@ -13,7 +13,7 @@
       e.preventDefault()
     })
 
-    // side bar
+    // back to top
     // setTimeout(function () {
     //   $('.bs-docs-sidenav').affix({
     //     offset: {
@@ -23,6 +23,10 @@
     //   })
     // }, 100)
 
+    setTimeout(function () {
+      $('.bs-docs-top').affix()
+    }, 100)
+
     // make code pretty
     window.prettyPrint && prettyPrint()