[Missing]: page about cron/systemd timers

### Page URL

n/a

### Select an option

- [ ] I found what I was looking for
- [x] I couldn't find what I was looking for
- [ ] I found the information, but it was incorrect
- [ ] I found the information, but it was incomplete
- [ ] I found the information, but it was confusing
- [ ] I encountered a technical issue (e.g., broken link, image not loading)

### Issue Description

As per title, we're missing a page on cron/systemd timers

(filed on behalf of cpaelzer for later discussion)

The issue names no repository file, test, or entry point. First define the intended Ubuntu Server documentation scope for cron and systemd timers, then identify where such a page belongs; done means an agreed page covering the requested topic.

Written by the indexing model from the issue text.