Update getting started steps to recommend beta

Until #205 lands, the steps for getting started on https://humanmade.github.io/webpack-helpers/guides/getting-started.html leave you with outdated versions of this library and associated Webpack tooling, which don't work in recent versions of Node. We should add a section within the getting started calling out to use
```sh
npm install --save-dev @humanmade/webpack-helpers@beta webpack@5 webpack-cli@4 webpack-dev-server@4
```
as the recommended version, to aid in compatibility with new project setup.