Update example configs and add certificate validation

[dcmcand]

Closes

Improves documentation and adds missing validation.

Description

• Update all example configs to include git_repository section with placeholder values
• Add certificate config to all examples showing selfsigned/letsencrypt options
• Add CertificateConfig.Validate() to require acme.email when type: letsencrypt
• Replace placeholder domain/repo URLs with generic examples
• Use slices.Contains for cleaner validation code

How to Test

1. Run tests: go test ./pkg/config/...
2. Try parsing example configs: ./nic validate -f examples/aws-config.yaml
3. Verify letsencrypt without email fails validation