[docs]: add checkpointing data

### URL/Location

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

### Describe What's Incorrect or Lacking

Missing details on what checkpoints on which operations

### Suggested Improvement

understand what to expect in execution history for each checkpoint.

understand different data formats/serialization formats for each sdk.

new page `state/checkpoint`.

### Additional Context

_No response_