[docs]: update plugin interface docs with new onOperationChange hooks

[nvasiu]

URL/Location

https://docs.aws.amazon.com/durable-execution/sdk-reference/observability/plugins/

Describe What's Incorrect or Lacking

The "Checkpoint-change hook" section is WIP for the Python and Java tabs currently:
https://docs.aws.amazon.com/durable-execution/sdk-reference/observability/plugins/

Suggested Improvement

Once this hook is implemented for those SDKs, this section needs to be updated with an example.

Additional Context

No response