Add to docs that plugin config can also come from babel plugin config

```
https://github.com/kentcdodds/babel-plugin-macros/blob/master/other/docs/user.md#config
```

The following is valid:

_babel.config.js_

```js
module.exports = {
presets: [require.resolve('@docusaurus/core/lib/babel/preset')],
plugins: [
[
'babel-plugin-macros',
{
twin: {
preset: 'styled-components',
},
},
],
],
}
```

Start with the Config section in other/docs/user.md, linked in the issue, and compare it with the babel.config.js example provided here. Update the documentation to explain that plugin configuration can be supplied through the Babel plugin config, including the shown twin preset example; the docs should clearly describe this valid configuration path.

Written by the indexing model from the issue text.