Moved docs/comparison.rst to secondary/docs/

Change-Id: I2281cb9efc2076506480734ae2afd4e6a8de1e69
Reviewed-on: http://review.couchbase.org/43471
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>