EE: Document the "Abuse reports" feature

What does this MR do?

CE: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22932

Add documentation for both admins (/user/admin_area/abuse_reports.md) and non-admins (/user/abuse_reports.md) to help them use the abuse reports feature

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?