Add styles for rustdoc mobile-topbar

Offset it by the size of the docs.rs topbar.

Author: jsha

templates/style/rustdoc-common.scss

@@ -101,6 +101,7 @@ div.rustdoc {
         }
     }
 
+    .mobile-topbar,
     #source-sidebar {
         top: $top-navbar-height;
     }