doc/netlink: Document the netlink-raw schema extensions

Add a doc page for netlink-raw that describes the schema attributes
needed for netlink-raw.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/r/20230825122756.7603-5-donald.hunter@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>