Add migration guide to convert from classic to Sandy plugins

Summary: Added migration guide from classic to Sandy plugins

Reviewed By: passy

Differential Revision: D27908674

fbshipit-source-id: d4b5a0b45e314226825375d762902041f90c04c8

website/sidebars.js

@@ -115,6 +115,7 @@ module.exports = {
       'extending/debugging',
       ...fbInternalOnly(['fb/adding-analytics-0']),
       'extending/plugin-distribution',
+      'extending/sandy-migration',
     ],
     'Desktop plugin APIs': [
       'extending/flipper-plugin',