Books without defined versions should get redirects for "stable", and "current"
Reported in this MM thread: https://chat.civicrm.org/civicrm/pl/nt7xmxes4fgk5m45cjcfg17xtc
When a book doesn't define any specific versions we set up one version based on the
master branch with
latest in the path. In this situation, we should also set up redirects for
current that take the reader to