System Diagram from Query DDL and Scan Request

yed and svg files for the 2 new system diagrams(Query Scan, Query DDL)
new markdown file system_query.md which explains the arrows (data/request flow)
linked the new markdown file from overview.md

Updated to reflect that Indexer always communicates with local Index Manager

Change-Id: I1f67103ec4c92e0c4b58066349e84e31106b4913
Reviewed-on: http://review.couchbase.org/34377
Tested-by: Sriram Melkote <siri@couchbase.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>
Reviewed-by: Sriram Melkote <siri@couchbase.com>