doc: net: Add yaml configuration information

Add documentation about the network stack configuration using
a yaml file.

Signed-off-by: Jukka Rissanen <[email protected]>

Author: jukkar

doc/connectivity/networking/index.rst

@@ -13,6 +13,7 @@ operation of the stacks and how they were implemented.
    overview.rst
    net-stack-architecture.rst
    net_config_guide.rst
+   network_config.rst
    networking_with_host.rst
    network_monitoring.rst
    network_tracing.rst