Simplify sidebar's overflow settings

Because sidebar needs to be scrollable, its overflow should default to auto.
Currently it's set to hidden and force individual elements to set overflow auto,
which overcomplicates things.

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

@@ -212,8 +212,8 @@ nav {
   position: fixed;
   top: 0;
   bottom: 0;
-  overflow: hidden;
   z-index: 10;
+  overflow: auto;
 
   /* Layout */
   width: var(--sidebar-width);
@@ -228,11 +228,6 @@ nav[hidden] {
   display: none;
 }
 
-nav #project-metadata {
-  overflow: auto; /* Make the content scrollable */
-  flex: 1; /* Take up remaining space */
-}
-
 nav footer {
   padding: 1em;
   border-top: 1px solid #ccc;
@@ -286,7 +281,6 @@ nav .nav-section {
   margin-top: 2em;
   border-top: 2px solid #aaa;
   font-size: 90%;
-  overflow: hidden;
 }
 
 nav h2 {