[Docs]: Document DURABLE_EXECUTION_TIME_SCALE env var

### Issue

I was surprised to learn about DURABLE_EXECUTION_TIME_SCALE, as I hadn't seen it documented anywhere. I think this would be useful to document if it's intended to be public.

### Page/Location

https://github.com/search?q=repo%3Aaws%2Faws-durable-execution-sdk-python-testing%20%20DURABLE_EXECUTION_TIME_SCALE&type=code

### Suggested Fix

_No response_