docs: networking: Add PPE driver documentation for Qualcomm IPQ9574 SoC

Add description and high-level diagram for PPE, driver overview and
module enable/debug information.

Signed-off-by: Lei Wei <quic_leiwei@quicinc.com>
Signed-off-by: Luo Jie <quic_luoj@quicinc.com>
Link: https://patch.msgid.link/20250818-qcom_ipq_ppe-v8-2-1d4ff641fce9@quicinc.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>