The source for Planck is hosted on GitHub: https://github.com/planck-repl/planck.
If you'd like to contribute, check it out, and take a look at the open issues list: https://github.com/planck-repl/planck/issues.
This documentation is also hosted in the Planck GitHub repository, under the site directory of that repository. So, if you find improvements that can be made, feel free to submit PRs!