Document input and output STAC item timestamps

[ngachung]

From ASIPS team, "I'm also curious about what is up with the properties. there are two fields there datetime and updated. is datetime when the file was written and then updated when it went into the catalog?
also, it looks like data_create_time is being set to data_begin_time if not specified?"

Capture where input timestamps from the STAC item is stored in the catalog.
Capture where the output timestamps in the STAC item is coming from.