Document Usage Quotas group settings page

What does this MR do?

Document the new Usage Quotas group settings page.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Links

• closes gitlab-org/gitlab-ce#63022