Add git abuse rate limit settings documentation

What does this MR do and why?

This adds documentation for the Git abuse rate limit feature.

Preview pages:

• http://main-ee-92558.35.193.151.162.nip.io/ee/user/admin_area/settings/#reporting
• http://main-ee-92558.35.193.151.162.nip.io/ee/user/admin_area/reporting/git_abuse_rate_limit.html

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/365907

MR acceptance checklist

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