You can onboard new hires through the Slack bot or through the web portal. ChiefOnboarding is available in multiple languages and uses timezones (even per new hire - for remote companies).
It used to be a SaaS, but a few months ago, I decided to make it completely open source. I still provide hosting for customers, but you can deploy it on your own servers if you want (docker-compose file included for easy deployment).
Documentation (under 'How to/Demo' there is a transcript of a demo with screenshots and all): https://docs.chiefonboarding.com
Source code: https://github.com/chiefonboarding/chiefonboarding
Let me know if you have any questions/feedback!