Removed void intro page to the tutorial

Summary: The 'extending flipper' introduction page didn't contain anything meaty, so collapsed it into the tutorial introduction, cleaning up the top level navigation.

Reviewed By: passy

Differential Revision: D25531718

fbshipit-source-id: ca0f98186e889e13cb97be8818db3588738e5039

website/docusaurus.config.js

@@ -41,7 +41,7 @@ const siteConfig = {
           position: 'right',
         },
         {
-          to: 'docs/extending/index',
+          to: 'docs/tutorial/intro',
           label: 'Extending',
           position: 'right',
         },

website/sidebars.js

@@ -52,7 +52,6 @@ module.exports = {
     Advanced: ['custom-ports', 'stetho'],
   },
   extending: {
-    'Extending Flipper': ['extending/index'],
     Tutorial: [
       'tutorial/intro',
       'tutorial/ios',