Common platform-dependent bug

[Songtech-0912]

First, thanks for working so hard on Jupyterbook v2! I just found an issue when upgrading from V1 to V2 for one of my books, so I hope it is helpful to mention here (for other users and MyST developers). If you are using Windows, the automatic upgrade will insert backslashes into the autogenerated myst.yml rather than forward slashes, which means that if someone else clones your book on Mac/Linux the build will fail (since backslashes are not valid file paths on Mac/Linux). Having banged my head against the wall trying to locate the source of this issue for the past 24 hours, I would love to see this fixed!

More details: jupyter-book/jupyter-book#2599