Update cron triggered runs docs

**Description**

We need to update the `Cron triggered runs docs` to make it easier to understand  how cron schedule work in OpenFn. 

<img width="1544" height="1117" alt="Image" src="https://github.com/user-attachments/assets/ee6b3cb9-5acd-4cf8-a4be-d17bc5c06767" />

[See docs here](https://docs.openfn.org/documentation/jobs/state#cron-triggered-runs)