Remove JSXTransformer, react-tools from website

[zpao]

There are a few pieces in flight here so putting this together to track for my own sanity

• Recommend Babel when we talk about transforming (Update tooling docs to suggest babel #4546, already landed on 0.13-stable)
• Remove our own use of JSXTransformer for transforming on site (Replace JSXTransformer with babel/browser #4131)
• Redirect jsx-compiler to babel repl (and remove our links to jsx-compiler)
• Update tutorial here and in tutorial repo (Use Babel to transform JSX reactjs/react-tutorial#71)
• Land all of this back on 0.13-stable ([docs-0.13] Use babel for transforming #4675)