Documentation: ABI: testing: document the new cpuidle sysfs file

Mention the new 'intel_c1_demotion' sysfs file in the "cpuidle" section
and refer to "Documentation/admin-guide/pm/intel_idle.rst" for more
information.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Link: https://patch.msgid.link/20250317135541.1471754-4-dedekind1@gmail.com


Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>