#857 docs: warn that the Python sandbox is unsafe

The Python data source documentation currently says it is disabled for security but does not explicitly warn that its sandbox is unsafe. Add a prominent warning before setup stating that we do not consider the sandbox a security boundary, do not recommend this data source with untrusted users, and disable it by default for that reason.

Remove the claim that the allowed built-ins are considered safe so the configuration guidance is consistent with the warning.

Validation: git diff --check passed. Documentation-only change; no runtime tests run.