It is typically found in the back matter, in non-OER textbooks and open resources alike. I’ve found many examples to confirm the same. I’d suggest placing it in the back matter, as is typical, and perhaps making it very clear in your introduction to the series that this is a collaborative effort. I do think it’s important for those accessing the book to know who all were involved in the making of the book: not just the team of authors and editors, but formatters, proofreaders, copyeditors, and more. So your introduction could point to this ‘About the Contributors’ page as well as the ‘Acknowledgements,’ perhaps, where we could publicly recognize all those involved!