Pages do not deploy when choosing a different publish_dir from `book`

[jbusecke]

I do not have a MRE yet, but wanted to post this as a reminder to get to this later (please feel free to assign me). I recently struggled with an issue that nothing was deployed to the gh-pages branch of a repo, but it magically fixed itself when I renamed the directory with my jupyter book content to books (from docs) (see the commit-diff that magically fixed it here).

I am not sure if I just did not switch the default somewhere, or if there is a bug somewhere that hardcodes the path. Ill try to get back to this in a bit but wanted to raise this as a reminder.