All you need to do for testing out a beta release of @pnp/spfx-controls-react is to install the dependency as follows:
npm install @pnp/spfx-controls-react@next --save
The control documentation is only live for public releases, not for beta versions. If you want to checkout the markdown files of all controls in the dev branch: beta documentation.
Once you installed the beta version, you can start using the controls in your solution. Go to the homepage to get an overview of all the available controls and the steps to get started: home.