[ruby/rdoc] Scrollable sidebar

This change makes the sidebar scrollable via `position: sticky` and `overflow: auto`;
See also <https://caniuse.com/?search=sticky>

https://github.com/ruby/rdoc/commit/4d52e24840

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

@@ -186,6 +186,10 @@ nav {
   font-family: Helvetica, sans-serif;
   font-size: 14px;
   border-right: 1px solid #ccc;
+  position: sticky;
+  top: 0;
+  overflow: auto;
+  height: calc(100vh - 100px); /* reduce the footer height */
 }
 
 main {