Docs for running periodic jobs

[jsierles]

We have nothing built-in for running periodic jobs. People ask a lot because other platforms offer it. But we can still guide people.

We have a simple example on how to run cron jobs. But it's not that useful on its own. It would be useful for those willing to add it to Dockerfile.

We should hat-tip to other good job tools like:

• sidekiq-cron (Ruby)
• node-cron (node)
• oban (elixir)

These tools could be documented more extensively on runtime-specific doc pages.

[catflydotio]

I'm closing this issue for Docs because it's framework-specific.

[bradgessler]

I wrote a framework agnostic guide for cron at https://fly.io/docs/app-guides/supercronic/

[catflydotio]

Even better. Closed as actually completed :)