Rate limit documentation for non-configurable limits

What does this MR do?

We have some rate limits that are not configurable or documented. There didn't seem to be a good place to put this information, since other limits are configurable and need their own page, so I have added a new section for these limits to make sure they are documented.

Screenshots

https://docs.gitlab.com/ee/user/gitlab_com/index.html#gitlabcom-specific-rate-limits

https://docs.gitlab.com/ee/security/rate_limits.html

Related issues

#207858 (closed)

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