docs/mdbook: move description to bottom

Long descriptions for plugins will shove the plugin's source link and
maintainer information far down the page. Since those are fairly short
and the description is a variable size. Moving below to maintain
consistent placement.

docs/mdbook/default.nix

@@ -66,11 +66,17 @@ let
         # Make sure this path has a valid info attrset
         if info ? file && info ? description && info ? url then
           "# ${lib.last path}\n\n"
-          + (lib.optionalString (info.description != null) "${info.description}\n\n")
           + (lib.optionalString (info.url != null) "**URL:** [${info.url}](${info.url})\n\n")
           + (lib.optionalString (
             maintainers != [ ]
           ) "**Maintainers:** ${lib.concatStringsSep ", " maintainersNames}\n\n")
+          + lib.optionalString (info.description != null) ''
+
+            ---
+
+            ${info.description}
+
+          ''
         else
           null;
     };