nixos docs: syntax highlight

- Rectifies diverging CSS by combining
   nixos/nixpkgs docs CSS
 - Moves our custom Highlight.js loader in to
   the hljs package
 - Switches the nixos docs to use SVG
   callouts too

pkgs/misc/documentation-highlighter/loader.js

@@ -0,0 +1,7 @@
+/* This file is NOT part of highlight.js */
+document.onreadystatechange = function () {
+    var listings = document.querySelectorAll('.programlisting, .screen');
+    for (i = 0; i < listings.length; ++i) {
+        hljs.highlightBlock(listings[i]);
+    }
+}