Index planner should be documented

Index planner details are not documented as of today. A lot goes on under the hood while placing an Index on the most suitable index node during index creation. It is necessary to document this. More details can be found at the blog https://blog.couchbase.com/index-planner-for-global-secondary-indexes/

In addition, Greedy Planner for index creation is targeted to be GA in 7.0.2. Would be great cover it in documentation as well. Design Spec at https://docs.google.com/document/d/1EMOEkS9vl5FOJ1bNDliNcfl0h5bYJ1MzCXTTeL1_E0c/edit#heading=h.z4opc2uc72bi

Cc Amit Kulkarni