Describe the prprocessor and its workflows

[ekohl]

Splitting off from #1679.

Today a lot of workflows rely on the prprocessor for automation, but few people know exactly how it works.

The best description is one for "end users", or in other words: Foreman developers who just want to benefit from the prprocessor. This should also include the project maintainers (like I maintain plugin X and have Redmine integration, how do I set this up).

The prprocessor developers description is considered out of scope for this issue.