Move react dependencies to docs.

[justin808]

This commit removes React dependencies, replacing them with docs.

Here's what changes in my example project with this change:

https://github.com/shakacode/react_on_rails_tutorial_with_ssr_and_hmr_fast_refresh/pull/21/files

I feel that if we're not going to support HMR, then it makes sense to let React configuration be outside of rails/webpacker.

[justin808]

@guillaumebriday @dhh Should we remove all of React from rails/webpacker?

[justin808]

@guillaumebriday this is rebased on master.

[justin808]

I've got a draft NPM package that will make React with SSR and HMR a one-liner for rails/webpacker users: shakacode/react_on_rails_demo_ssr_hmr#24

Here's the config that takes the docs change and puts it into the NPM module: https://github.com/shakacode/react-on-webpacker/blob/main/babel/preset.js