Move `usage_quotas.md` to `storage_usage_quotas.md`

What does this MR do and why?

Move usage_quotas.md to storage_usage_quotas.md

This page only documents storage usage quotas, so in order to avoid other tabs of Usage Quotas Page to link to this doc, we're renaming the file to be more specific. We're also updating the doc's references in the code.

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