Document bigcouch ram requirements

we ran into an issue with bigcouch and too little ram in the past, and now again on couch nodes with 512mb (see #6440 (closed)).

We should document the ram requirements for couch nodes. so far, we only have a small section on general node requirements, from https://leap.se/en/docs/platform/quick-start#Requirements:

"If you allocate 2 CPUs and 8G of memory to each node, that should be more than enough to begin with."

(from redmine: created on 2014-11-26, closed on 2016-05-26, relates #6440 (closed), relates #5088 (closed), relates #5011, relates #5084, relates #5076, relates #5024, relates #4932 (closed), relates #4935 (closed), relates #4906 (closed), relates #6644)