docs: add troubleshooting decision tree for common scan failures

[mldangelo]

Description

Create a troubleshooting guide that helps users diagnose common scan failures. The README has a brief troubleshooting section, but a more structured decision tree would help users self-serve.

Common issues to cover

• modelaudit doctor reports missing optional dependencies
• Scan exits with code 2 (error) — common causes
• Permission errors when scanning remote sources (HF Hub, S3, GCS)
• Large file timeouts or memory issues
• Scanner not detecting a known format (wrong file extension, missing extra)

Suggested location

docs/user/troubleshooting.md — new file, linked from README.

References

• modelaudit doctor --show-failed output
• Exit codes: 0 (clean), 1 (issues found), 2 (scan error)
• README.md troubleshooting section

Acceptance criteria

• Covers at least 5 common failure scenarios
• Each scenario has: symptom, cause, fix
• Links to relevant CLI commands (modelaudit doctor, etc.)