Add a section on object storage specifics

What does this Merge Request do?

This adds an additional section to the Dependency Proxy docs with details on how to configure when using object storage.

Merge Request checklist

• [-] Tests added for new functionality. If not, please raise Issue to follow-up.
• [-] This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• [-] gdk doctor test added, if needed.