Add new rate limits for GitLab.com

What does this MR do and why?

Adding additional documentation for the new rate limits that will apply to GitLab.com moving forward. Related to this comment.

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• https://docs.gitlab.com/ee/security/rate_limits.html
• https://docs.gitlab.com/ee/administration/settings/rate_limit_on_projects_api.html
• https://docs.gitlab.com/ee/administration/settings/rate_limit_on_groups_api.html

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.