Documentation: net: move device drivers docs to a submenu

Some of the device drivers have really long document titles
making the networking table of contents hard to look through.
Place vendor drivers under a submenu.
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Dave Watson <davejwatson@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/device_drivers/index.rst

+.. SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+
+Vendor Device Drivers
+=====================
+
+Contents:
+
+.. toctree::
+   :maxdepth: 2
+
+   freescale/dpaa2/index
+   intel/e100
+   intel/e1000
+   intel/e1000e
+   intel/fm10k
+   intel/igb
+   intel/igbvf
+   intel/ixgb
+   intel/ixgbe
+   intel/ixgbevf
+   intel/i40e
+   intel/iavf
+   intel/ice
+
+.. only::  subproject
+
+   Indices
+   =======
+
+   * :ref:`genindex`

Documentation/networking/index.rst

@@ -11,19 +11,7 @@ Contents:
    batman-adv
    can
    can_ucan_protocol
-   device_drivers/freescale/dpaa2/index
-   device_drivers/intel/e100
-   device_drivers/intel/e1000
-   device_drivers/intel/e1000e
-   device_drivers/intel/fm10k
-   device_drivers/intel/igb
-   device_drivers/intel/igbvf
-   device_drivers/intel/ixgb
-   device_drivers/intel/ixgbe
-   device_drivers/intel/ixgbevf
-   device_drivers/intel/i40e
-   device_drivers/intel/iavf
-   device_drivers/intel/ice
+   device_drivers/index
    dsa/index
    devlink-info-versions
    ieee802154