doc/netlink: Describe nested structs in netlink raw docs

Add a description and example of nested struct definitions
to the netlink raw documentation.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20240129223458.52046-12-donald.hunter@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>