Documentation follow-up

[Timer]

Copied from #5141:

• 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 (Babel JSX Syntax error when importing from a linked library #5100)
• Check and document Apollo (Checkout apollo compatibility #5126) and Relay support
• MDX Using mdx in @next  #5149

[tobiasbueschel]

Hey @Timer 👋

Just noticed that this issue is still open - as most items relate to either 1.x or 2.x and are from late 2018, shall we close this issue? Or is there anything left? :)