Fix footer display

Currently, sidebar's height is reduced to avoid covering up the footer.
But this cuts the sidebar short even before the footer is reached.
Alternatively, we can achieve the same effect by increasing the footer's
z-index to make it appear on top of the sidebar.

Author: st0012

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

@@ -214,13 +214,6 @@ nav {
   background: white;
 }
 
-@media (min-width: 1024px) {
-  nav {
-    min-height: unset;
-    height: calc(100vh - 100px); /* reduce the footer height */
-  }
-}
-
 main {
   display: block;
   margin: 3em auto 1em;
@@ -731,4 +724,8 @@ pre {
   font-family: "Source Code Pro", Monaco, monospace;
 }
 
+footer {
+  z-index: 20;
+}
+
 /* @end */