docs: networking: device drivers: add can sub-folder

Add the container for CAN drivers documentation.

Link: https://lore.kernel.org/all/20220107193105.1699523-7-mkl@pengutronix.deSigned-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

Documentation/networking/device_drivers/can/index.rst

+.. SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+
+Controller Area Network (CAN) Device Drivers
+============================================
+
+Device drivers for CAN devices.
+
+Contents:
+
+.. toctree::
+   :maxdepth: 2
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`

Documentation/networking/device_drivers/index.rst

@@ -11,6 +11,7 @@ Contents:
    appletalk/index
    atm/index
    cable/index
+   can/index
    cellular/index
    ethernet/index
    fddi/index