Map features in high-level documentation #19

In order to help guiding devs (and possibly AI agents), expand on "docs/reference.md", with something like "design.md" or "specification.md", covering the main modules and features. Think of it a document you would want to have in your hands to reimplement regzbot.

That should help to abstract the current state of regzbot, allowing us to focus on what we want to extract in the form of a library more clearly.