docs: fix scroll-y on sidebar (#1653)

The documentation cannot be scrolled when the content bigger than
container. Add `overflow-y: auto;` fix this problem.

Author: siarie

docs/css/index.css

@@ -25,6 +25,7 @@ body > nav {
   background: #F4F4F4;
   font-size: 15px;
   padding-right: 10px;
+  overflow-y: auto;
 }
 
 nav > ul {
@@ -120,4 +121,4 @@ p {
 h1#pdfkit + h2 {
   margin-top: 0;
   font-size: 1.2em;
-}
+}