Where to advertise a postdoctoral position to widen the diversity of applicants?
https://jobrxiv.org/ and twitter although I don't know how much math people use either. For my bio-related field they're pretty good.
You'll probably get plenty of diversity in your applicant pool but you'll end up removing most of it when you filter for prestige PhD institutions and advisors. You should try to remember that URMs from second tier PhD institutions are just as smart in aggregate as the population from Stanford or whatever, but are likely to publish worse and have fewer opportunities. Evaluate them based on who's outperformed for their grad environment and in general you'll probably have a bunch of applicants you wouldn't have otherwise considered. (This is also useful for identifying the halfwits from Harvard who have underperformed despite every possible advantage - don't hire them).