user guide: add page navigation links

rb/assets/user_guide.html.erb

@@ -28,6 +28,19 @@
       a {
         text-decoration: none;
       }
+      #page_nav {
+        margin-top: 2em;
+      }
+      #page_nav_prev {
+        float: left;
+      }
+      #page_nav_toc {
+        text-align: center;
+      }
+      #page_nav_next {
+        float: right;
+        text-align: right;
+      }
       .code {
         padding-left: 2ex;
       }