new sub-folder 'Building a Desktop Plugin' (Creating Plugins)

Summary:
Major changes to js-custom.mdx

The content for the following pages is moved to the new sub-folder 'Building a Desktop Plugin':
* js-custom.mdx
* js-setup.mdx
* js-table.mdx

The Title and Sidebar labels have been changed accordingly.

Changes made to sidebar.js,

Reviewed By: lblasa

Differential Revision: D36415874

fbshipit-source-id: 8f1634abc1459d1905da62a1b76bac4b621d0da5

website/sidebars.js

@@ -96,9 +96,13 @@ module.exports = {
       'tutorial/android',
       'tutorial/react-native',
       'tutorial/javascript',
-      'tutorial/js-setup',
-      'tutorial/js-table',
-      'tutorial/js-custom',
+      {
+        'Building a Desktop Plugin': [
+          'tutorial/js-setup',
+          'tutorial/js-table',
+          'tutorial/js-custom',
+        ],
+      },
       'tutorial/js-publishing',
     ],
     'Development workflow': [