Document general pseudonyms

The original idea was to move the documentation from castellum!1911 (e4f25865) to this repo. However, the stuff I wrote for this repo ended up being considerably different from that. I see no harm in keeping the short conceptual overview in the castellum repo.

In some places I sprinkled paragraphs related to general domains into existing guides. Since general domains are an optional feature that is hidden if no general domains exist, this might be confusing for some users.