net/mlx5: Consolidate devlink documentation in devlink/mlx5.rst

De-duplicate documentation by removing mellanox/mlx5/devlink.rst. Instead,
only use the generic devlink documentation directory to document mlx5
devlink parameters. Avoid providing general devlink tool usage information
in mlx5-specific documentation.

Signed-off-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>