I've used a newbie notes file for years. Mostly written but screenshots and video if they make sense. Preferably in something version controlled.
One of the first tasks of any new joiner is to update the notes with any errors or omissions. That should mean the notes are only as outdated as the last person to join. Which if you have a steady cadence of people joining shouldn't be too bad.
Having a buddy to answer any questions is good. Budget that taking 30% of that person's time at least initially.