CE: Document the "Abuse reports" feature

What does this MR do?

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.

Closes #45552 (closed)