[FAB-4213] Update doc with new fabric-ca doc link

The documentation for fabric-ca was moved over from the
fabric repository to the fabric-ca repository. A new
readthedocs page was setup. The doc in fabric repo needs
to be updated with a link that points to the new location
for the fabric-ca documentation.

Change-Id: I4b256af88160b6005932decc39a2ca4cd3823d52
Signed-off-by: Saad Karim <[email protected]>

Author: Saad Karim