This is usually why new starters are taken out for lunch or shown the ropes by members of their team. If no one appears to have offered, there's nothing wrong with doing so yourself, or suggesting it to someone else.
The informal lunch setting, sometime during the first week, is the perfect time to bring up the things you won't find written down anywhere. Unwritten rules demand an informal atmosphere, after all.
That's the unofficial approach. If, on the other hand, you want to smoothen the onboarding process, have a look at Matthew Gilliard's Supporting a new starter, specifically the Onboarding buddy section:
Goal: Make sure the initial onboarding instructions are accurate and provide help with things which starters need to do only once.
The onboarding buddy is usually expected to be the previous most recent new starter (who has it all freshest in their memory). I’ve worked with this arrangement before and it’s helpful for keeping things up-to-date.
edit Incorporating Chris H's comment:
Sad to say, if the new starter is a member of an under-represented or under-valued group , it may be worth having someone with similar experiences support them. As a man who's worked in male-dominated fields, for example, with the best will in the world there are gaps in how I could support women