Add scroll-margin-top on all :target elements

Author: omegahm

lib/rdoc/generator/template/darkfish/css/rdoc.css

@@ -84,6 +84,7 @@ h5:target,
 h6:target {
   margin-left: -10px;
   border-left: 10px solid #f1edba;
+  scroll-margin-top: 1rem;
 }
 
 /* 4. Links */