Documentation: tracing: Add PCI tracepoint documentation

Documentation: tracing: Add PCI tracepoint documentation

The PCI tracing system provides tracepoints to monitor critical hardware
events that can impact system performance and reliability. Add
documentation about it.

Signed-off-by: Shuai Xue <xueshuai@linux.alibaba.com>
[bhelgaas: squash fixes:
https://lore.kernel.org/r/20260108013956.14351-2-bagasdotme@gmail.com
https://lore.kernel.org/r/20260108013956.14351-3-bagasdotme@gmail.com]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://patch.msgid.link/20251210132907.58799-4-xueshuai@linux.alibaba.com