Guidance for scheduled jobs needs updating

What has been written to date is mostly still correct but the guidance could be expanded to include log and metric isolation as a key reason to favour jobs compute clusters over all-purpose compute clusters for scheduled jobs. Additionally, the instance pools feature is referenced as something that '_will_' reduce startup time for short, frequent jobs - this has since been released into GA and should be reflected as such 🙂

Locate the existing guidance for scheduled jobs and review the sections covering jobs compute clusters and instance pools. Update the guidance to include log and metric isolation as a reason to prefer jobs compute clusters, and reflect that instance pools are generally available rather than planned.

Written by the indexing model from the issue text.