docs: add cache behavior and local data handling documentation

[mldangelo]

Description

Document ModelAudit's caching behavior, data retention, and local file handling for users who need to understand what data ModelAudit stores locally.

Topics to cover

• What gets cached (scan results, downloaded models)
• Where cache files are stored (platformdirs location)
• Cache expiration and invalidation
• How to clear the cache
• What telemetry data is collected (and how to opt out)
• File handling during scans (read-only, no modification of scanned files)

References

• `modelaudit/cache/` — caching implementation
• `modelaudit/telemetry.py` — telemetry implementation
• README.md telemetry section
• `platformdirs` documentation for cache directory locations

Suggested location

`docs/user/data-handling.md` — new file.

Acceptance criteria

• Documents cache location for each platform (Linux, macOS, Windows)
• Documents opt-out mechanisms for telemetry
• Confirms ModelAudit never modifies scanned files