Add "Example" label to example code blocks

The label should help users distinguish example code blocks from other
code blocks, such as method source code.

It's only applied to Ruby code examples.

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

@@ -601,6 +601,24 @@ main .method-source-code.active-menu {
   max-height: 100vh;
 }
 
+main pre.ruby::before {
+  content: "Example";
+  color: var(--secondary-color);
+  border-bottom: 1px solid var(--border-color);
+  display: block;
+  padding: 10px 0;
+  font-size: 0.9em;
+  margin-bottom: 10px;
+}
+
+main pre.ruby {
+  margin: 0;
+  padding: 0px 16px 16px 16px;
+  overflow-x: auto;
+  border-radius: 4px;
+  border-color: var(--border-color);
+}
+
 main .method-description .method-calls-super {
   color: var(--text-color);
   font-weight: bold;