Add PFA family-case overlap explainer

[culpen90]

Summary

• Add a Protection From Abuse and family-case overlap explainer for operators and reviewers.
• Document why overlap should trigger independent family-case analysis review instead of importing PFA outcomes into family-case conclusions.
• Link the explainer from user, reviewer, and attorney-reviewer guides.
• Add regression coverage for the explainer's core flags and guide links.

Validation

• python3 scripts/check-doc-unsafe-claims.py
• Manual execution of the new doc test assertions with python3
• git diff --check
• python -m pytest: 575 passed, 1 skipped in 44.22s

Full pytest was run in an external virtual environment with pytest 9.1.1, fastapi 0.115.14, and starlette 0.46.2.

Closes #10