Whether you have direct push access to the repo or are planning to submit a pull request for some content changes, the issues list is a good place to start. If there is an issue that interests you, comment on the issue and sign up for it. If you want to add some new content or modify existing content that doesn't have an associated issue, please file an issue so we can keep track and go for it.
Check out https://docs.hhvm.com/hack/contributing/introduction to learn how to run a developer instance of this site and make changes.