Document possible Babel customizations

I'm building a package with `@emotion/react` and need to include their babel-plugin.

I've read on some issues here that Babel customization is possible with a `.babelrc` file, but it is not documented anywhere. And is the Babel configuration getting merged in or replaces the default one?

I'd also write the docs for that if someone can point me in the right direction :)

Start by reviewing the existing documentation and how the `.babelrc` file is handled. Verify whether custom Babel settings are merged with or replace the defaults, and determine how an `@emotion/react` Babel plugin is configured. Done means the supported customization behavior and an example are documented.

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