add kvscheduler diagram to plugin/kvscheduler

[chrismetz09]

diagram at https://github.com/ligato/vpp-agent/blob/master/docs/kvscheduler/kvscheduler.md improves understanding

[ondrej-fabry]

This is actually already in the docs: https://docs.ligato.io/en/latest/developer-guide/kvscheduler/#diagram

However we should probably add some links in there.

Also we might need to rethink all the docs for KVScheduler.. because it seems three sections now contain KVScheduler.. seems there is some duplication there as well..

[chrismetz09]

apologies again - in multitask overload including jumping around between different doc kvsched items. Meant to say diagram should be included in tutorials/kvs section of docs. Perhaps that one posted to slack week before last. #20 should address this I guess.