Documentation: networking: add Twisted Pair Ethernet diagnostics at OSI Layer 1

Documentation: networking: add Twisted Pair Ethernet diagnostics at OSI Layer 1

This patch introduces a diagnostic guide for troubleshooting Twisted
Pair  Ethernet variants at OSI Layer 1. It provides detailed steps for
detecting  and resolving common link issues, such as incorrect wiring,
cable damage,  and power delivery problems. The guide also includes
interface verification  steps and PHY-specific diagnostics.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://patch.msgid.link/20241004121824.1716303-1-o.rempel@pengutronix.de
Signed-off-by: Paolo Abeni <pabeni@redhat.com>