docs: networking: device drivers: can: add ctucanfd to index

This patch adds the ctucanfd-driver document to the index.

Link: https://lore.kernel.org/all/e4396244da6b008c671def9f50bb983a10389863.1650816929.git.pisa@cmp.felk.cvut.cz


Signed-off-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>
[mkl: split into separate patches]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>