docs: networking: convert caif files to ReST

There are two text files for caif, plus one already converted
file.

Convert the two remaining ones to ReST, create a new index.rst
file for CAIF, adding it to the main networking documentation
index.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>