Add sub titles on storage usage quotas

What does this MR do and why?

This partially implements https://gitlab.com/gitlab-org/gitlab/-/issues/378319+

Here we're adding Namespace Overview and Storage usage breakdown sub titles as we have in the design here https://gitlab.com/gitlab-org/gitlab/-/issues/372093/designs/storage_page_redesign.png and moving the Buy storage button up.

Screenshots or screen recordings

Before	After

How to set up and validate locally

Go to any Group Usage Quotas Page (e.g. https://gdk.test:3443/groups/flightjs/-/usage_quotas#storage-quota-tab) or Profile Usage Quotas Page (https://gdk.test:3443/-/profile/usage_quotas#storage-quota-tab) and the sub titles should be as in the screenshots

MR acceptance checklist

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