Hi,
thank you for a valuable feedback. We are aware that our manual is ‘this is what you can do’ rather than ‘how to do x’ and there are other aspect of theforeman.org that need improvement. We are currently in a process of redesigning the pages and restructuring the information (more on that in a separate thread). Docs at theforeman.org, youtube videos, IRC channel and this forum are the main sources of information that we have.
I would say that the knowledge you need depends on what you want to achieve. Different people have different workflows and most people use only a subset of features. You do not need Puppet knowledge if you do not plan to use it.
What are your expectations of Foreman? What brought you to installing it in the first place? Is there a specific thing that you want Foreman to do for you?