docs: Add ABI documentation for intel_pmt feature directories

Add a new sysfs ABI documentation file describing the layout and content of
the features-<PCI BDF>/ directory used by Intel PMT (Platform Monitoring
Technology). This directory exposes telemetry and control feature details
for a given PMT PCI device.

Signed-off-by: David E. Box <david.e.box@linux.intel.com>
Link: https://lore.kernel.org/r/20250703022832.1302928-10-david.e.box@linux.intel.com


Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>