Update documentation for Usage quota

What does this MR do and why?

Contributes to #345478 (closed)

• Clariry the difference in permissions between Usage quota data and Usage quota page
• Fix permissions for Usage quota page

How to set up and validate locally

• http://localhost:3000/help/user/permissions.md#group-members-permissions
• http://localhost:3000/help/user/usage_quotas.md#storage-usage-statistics

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.