docs: provide recommendations for using rollout window to avoid hitting rate limits

[courier-new]

A customer recently bumped up against the GitHub API rate limit when they configured a rollout window that was much too aggressive (5/second).

Our docs talk about using rollout windows to avoid overwhelming CI systems, but we could make it clearer that you can also use them if you're hitting code host API rate limits, and provide recommendations for sufficiently-relaxed rates (e.g. "use a rate that is no faster than 1/minute") that would not exceed the rate of any one code host.

As noted in the support issue, GitHub also has an undocumented internal object creation limit that produces the "was submitted too quickly" error message, and has a 1 hour cooldown. We should also add a note to help with troubleshooting this, as it has likely impacted other customers before but only recently has had clarity from someone on the GitHub team about it.