Minor documentation updates

- Change Index Manager to Index Coordinator in execution flow diagrams and writeups
- Link overview.md from README
- Highlight IndexCoordinator in overview.md

Change-Id: Ib5e0e1877ad6945ecbf06cbce8d2895dfdf0ae31
Reviewed-on: http://review.couchbase.org/34985
Reviewed-by: John Liang <john.liang@couchbase.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>