Add documentation for namespace level bans

What does this MR do and why?

This adds documentation for the namespace level git rate limiting and adjusts documentation for application level git rate limiting.

Preview pages:

• https://main-ee-102671.docs.gitlab-review.app/ee/user/group/reporting/git_abuse_rate_limit.html (new)
• https://main-ee-102671.docs.gitlab-review.app/ee/user/admin_area/reporting/git_abuse_rate_limit.html (changed)

Issue: https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/58

MR acceptance checklist

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