Summary:
Some old welcome links are 'Page Not Found'.
## Changelog
Some old links are 'Page Not Found', so update them as follows:
1. Update "Using Flipper" link to 'https://fbflipper.com/docs/features'
2. Update "Add Flipper Support to Your App" link to 'https://fbflipper.com/docs/getting-started'
Pull Request resolved: https://github.com/facebook/flipper/pull/3902
Test Plan: Pass the test on a Mac.
Reviewed By: lblasa
Differential Revision: D37927376
Pulled By: passy
fbshipit-source-id: 0aac129932395e3f2ce3afa8c47cc637f3e814a8
bd5ac524f5