Documentation: networking: Add stmmac to device drivers list

Add the stmmac RST file to the index.
Signed-off-by: NJose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/device_drivers/index.rst

@@ -25,6 +25,7 @@ Contents:
    mellanox/mlx5
    netronome/nfp
    pensando/ionic
+   stmicro/stmmac
 
 .. only::  subproject and html