docs: add guidance for avoiding rate limits & GitHub's internal limit

Created by: courier-new

Closes https://github.com/sourcegraph/sourcegraph/issues/39773.

Adds guidance around avoiding hitting rate limits when using rollout windows, as well as what it means when you encounter GitHub's cryptic "was submitted too quickly" error (tl;dr sux, plz wait an hour).

I also filed a follow-up issue to maybe try to automatically respect this internal GitHub limit, as it's come up with a couple different customers and it's always confusing.

Test plan

Just a docs change.