[docs] mention logfile and remove exempt_group

[konstruktoid]

This PR:

• adds a note regarding logfile since it's a CIS Ubuntu Linux (e.g 24.04 LTS) Benchmark requirement (5.2.3 Ensure sudo log file exists)
• remove mentions of exempt_group since it's not supported

/etc/sudoers:10:10: unknown setting: 'exempt_group'
Defaults        exempt_group=sudo
                ^~~~~~~~~~~~
/etc/sudoers.d/012_logfile:1:10: unknown setting: 'logfile'
Defaults logfile="/var/log/sudo.log"