Merge troubleshooting pages

Summary:
This replaces the existing "FB Internals" troubleshooting" page with a redirect to the now single troubleshooting page, to make sure no links break.

It copies the content of that internal page into an internal-troubleshooting.mdx file, that isn't in any sidebars.

And it gets the main troubleshooting page to include that page, when doing an internal build. So now we only have one troubleshooting page.

Reviewed By: passy

Differential Revision: D25372552

fbshipit-source-id: 1a310ca1a4a21683d6074d480e819e5af40c8af2

website/sidebars.js

@@ -75,14 +75,9 @@ module.exports = {
       'extending/error-handling',
       'extending/testing',
       'extending/debugging',
-      ...fbInternalOnly([
-        'extending/fb/desktop-plugin-releases',
-      ]),
-    ],
-    'Deprecated APIs': [
-      'extending/ui-components',
-      'extending/js-plugin-api',
+      ...fbInternalOnly(['extending/fb/desktop-plugin-releases']),
     ],
+    'Deprecated APIs': ['extending/ui-components', 'extending/js-plugin-api'],
     // end-internal-sidebars-example
     'Other Platforms': [
       'extending/new-clients',
@@ -97,7 +92,6 @@ module.exports = {
   },
   'fb-internal': {
     'FB Internal': fbInternalOnly([
-      'fb/troubleshooting',
       'fb/release-infra',
       'fb/Add-flipper-to-android-app',
       'fb/Adding-flipper-to-ios-app',