docs: Add troubleshooting guide with common issues

[abtreece]

Summary

Create a comprehensive troubleshooting guide to help users diagnose and resolve common confd issues.

Proposed Content

1. Connection Issues

Backend connectivity problems:

• etcd: connection refused, timeout, TLS errors
• Consul: ACL denied, agent unavailable
• Vault: authentication failures, token expired, seal status
• Redis: connection reset, auth failed
• AWS (SSM/ACM/Secrets Manager): credential chain, region issues

2. Template Errors

• Template syntax errors and how to debug
• Missing keys behavior
• Type conversion issues (getv vs getvs)
• Template function errors

3. Permission Issues

• File ownership/mode problems
• check_cmd/reload_cmd permission denied
• SELinux/AppArmor considerations

4. Watch Mode Issues

• Watch not triggering updates
• High CPU usage in watch mode
• Backend-specific watch limitations

5. Common Misconfigurations

• Prefix vs keys confusion
• TOML syntax errors in conf.d/
• Path issues (relative vs absolute)

6. Debugging Techniques

• Using --log-level debug
• Using --noop for dry runs
• Checking staged files
• Template output inspection

7. Platform-Specific Issues

• Kubernetes: sidecar patterns, init containers
• Docker: signal handling, volume mounts
• systemd: service configuration, restart policies

Location

docs/troubleshooting.md