In docs/design.md#source-file-processors, document the rationale for using qjz9zk as TLD

Description

Explain the rationale in docs, to have git track the reasons together with their implementation.

Who's implementing?

• I'm willing to implement this feature myself

The problem

Using .qjz9zk as a TLD seens odd without explanation. The more intuitive way would seem to be to use a 2nd level domain qjz9zk.invalid. There had been an issue for this (#1681, locked without resolution) and a discussion (#1891, now deleted), so currently the reasons seem undocumented.

Possible solutions

Independent of the current state of the discussion, I think this decision is important enough to explain in the docs. That way, the git history will reflect the evolution of maintainers' opinion on that discussion. This will make it easier to validate whether a specific version's implementation properly matches the reasons at that time.

Alternatives

No response

Additional context

No response