Hey Chloe.
Does your company incorporate DEI as part of their general workplace process and policies? Not just at the onboarding stage? DEI cannot be summarised into one policy - it's something companies need to work at/on all the time.
If you have actual programmes and initiatives for your whole company, then at onboarding, they just need to be told about your DEI processes, and they can then continuously join in on what the rest of the company has access to. If your company doesn't actually have any DEI initiatives, then there isn't much more you can give to new starters - otherwise they may get something at onboarding that then doesn't translate to when they are working at the company.

In our company we have a DE&I council, these members are nominated by the business and they attend the monthly meetings wherein initiative and interventions are discussed and agreed upon.