Add documentation for DNS and Kerberos services

- Created a new markdown file for DNS service configuration, detailing installation, configuration, and testing steps.
- Created a new markdown file for Kerberos authentication service, including DNS configuration, installation, and user management.
- Added a new markdown file for virtualization commands related to DNS and Kerberos utilities, including usage examples for `dig`, `nslookup`, `kinit`, `klist`, `kdestroy`, `kpasswd`, and `kadmin`.