Container Registry: document general scaling recommendations

What does this MR do?

This update to the container registry documentation introduces a new section on scaling by component. It provides guidance on identifying potential performance bottlenecks and offers recommendations for scaling various components of the registry, including the database, registry server, Redis cache, and storage. Additionally, it includes information on adjusting online garbage collection settings and scaling horizontally with the registry server.

This MR adds a new section outlining how to scale the container registry. In this initial effort, we hope to provide a rough guide for self-managed admins on what techniques are available, without being prescriptive about when and how to apply these techniques. We may become more prescriptive in the future, but since registry usage can vary somewhat independently from GitLab usage, this would need to be informed by user feedback, with the changes here serving as a template.

Related issues

Original discussion: #480742 (comment 2076151030)

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