docs(chapter-33): warn contributors how to test safely

Chapter: 33 - Safe local development.

Plain-English map:
Document how to avoid using production databases, production buckets, real
client documents, or browser-exposed service-role keys during local testing.

Why it matters:
New contributors can accidentally leak sensitive data or admin credentials
while only trying to run the app. The docs should prevent that mistake early.

Principle:
Good setup docs are a security control.

Precedent borrowed:
Upstream PR #133.

Upstream base: willchen96/mike@d39f580.
Original local commit: 5e3608b.