net/mlx5: Add counter information to mlx5 driver documentation

Update rst file to contain general information about statistics counters
for the mlx5 driver. Add specifics about individual counters in list
tables.

Signed-off-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>

Documentation/networking/device_drivers/ethernet/mellanox/mlx5/index.rst

@@ -16,6 +16,7 @@ Contents:
    devlink
    switchdev
    tracepoints
+   counters
 
 .. only::  subproject and html