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 stable, and current that take the reader to latest.