Documentation follow-up

Created by: Timer

Copied from #5141 (closed):

• Update React's "Add React to Website" JSX section
• Prepare blog post
• Touch up migration guide from 1.x
• Make a decent migration guide from 2.x betas
  • Monorepo migration (https://github.com/facebook/create-react-app/issues/5100)
• Check and document Apollo (https://github.com/facebook/create-react-app/issues/5126) and Relay support
• MDX https://github.com/facebook/create-react-app/issues/5149