Update Sidekiq configuration when using Registry

What does this MR do?

• Updating recommendation for configuring the Container Registry when using an external Sidekiq. When providing registry_external_url, Registry service will be running on Sidekiq which is not required, see more information at gitlab-environment-toolkit#714 (comment 1489071375) and gitlab-environment-toolkit!1068 (comment 1501098622)
• Clean up Sidekiq configuration in Reference Architecture to use Sidekiq role https://gitlab.com/gitlab-org/omnibus-gitlab/-/blob/master/doc/roles/index.md#sidekiq-roles

Related issues

Registry running on Sidekiq nodes (gitlab-environment-toolkit#714 - closed)

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