document OL 2.0.0

[pawel-big-lebowski]

Document changes related to:

• Emit job and dataset runless metadata OpenLineage#1880

Need to doc:

• within schema 2.0.0, not only we accept RunEvent but also DatasetEvent and JobEvent
• different events are distinguished by schemaURL field and NOT by eventType. This may be misleading but eventType has been introduced earlier to distnguish start, success or fail event types (totally different context) and we don't want to change the existing behaviour.