Documentation: net: add flow control guide and document ethtool API

Introduce a new document, flow_control.rst, to provide a comprehensive
guide on Ethernet Flow Control in Linux. The guide explains how flow
control works, how autonegotiation resolves pause capabilities, and how
to configure it using ethtool and Netlink.

In parallel, document the pause and pause-stat attributes in the
ethtool.yaml netlink spec. This enables the ynl tool to generate
kernel-doc comments for the corresponding enums in the UAPI header,
making the C interface self-documenting.

Finally, replace the legacy flow control section in phy.rst with a
reference to the new document and add pointers in the relevant C source
files.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://patch.msgid.link/20250924120241.724850-1-o.rempel@pengutronix.de


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