Document the ratio of number of cbs to sg

First comment:

Filing this issue on behalf of @suravarjjala

There is no recommended configuration documentation of how many couchbase servers are needed per sync gateway or how many sync gateways can be handled by a server. This is a tracking ticket to add this info to the documentation.

see other comments: https://github.com/couchbaselabs/couchbase-mobile-portal/issues/736