DOC: Add more information about checkpointing and time travel

[riyavsinha]

Issue with current documentation:

No response

Idea or request for content:

It would be really helpful to have content to understand where/how often checkpointing is invoked during the graph execution lifecycle.

Then, it would be helpful to have documentation / a notebook example to show how to correspond a checkpoint's timestamp to a node's execution so that we can implement "retrying" from a certain node in the graph execution.