Change neovim.org references to neovim.io

contrib/doxygen/extra.css

@@ -422,7 +422,7 @@ th {
   padding: 0;
 }
 
-/* neovim.org/css/main.css */
+/* neovim.io/css/main.css */
 
 * {
   -moz-box-sizing: border-box;

contrib/doxygen/header.html

@@ -24,7 +24,7 @@ $extrastylesheet
   <div class="container">
     <nav class="site-nav">
       <ul>
-        <li><a href="http://neovim.org/">Neovim Home</a></li>
+        <li><a href="http://neovim.io/">Neovim Home</a></li>
       </ul>
     </nav>
     <a href="$relpath^index.html">