docs: add cron-jobs documentation

Description

This MR adds the missing documentation for Cron jobs in Grove.

Supporting information

https://tasks.opencraft.com/browse/SE-6041

Testing instructions

Steps to test the changes:

1. Read the documentation.

Dependencies

NA

Screenshots

NA

Checklist

If any of the items below is not applicable, do not remove them, but put a check in it.

• All providers include the new feature/change
• All affected providers can provision new clusters
• Unit tests are added/updated
• Documentation is added/updated
• The TOOLS_CONTAINER_IMAGE_VERSION in ci_vars.yml is updated
• The grove-template repository is updated

Additional context

NA