user guide: add rscons logo

rb/assets/user_guide.html.erb

@@ -31,6 +31,11 @@
       .separator {
         height: 2em;
       }
+      .img_block_center {
+        display: block;
+        margin-left: auto;
+        margin-right: auto;
+      }
       .page_nav {
         width: 100%;
         table-layout: fixed;