Update step to copy secrets to sidekiq nodes in ref architectures

What does this MR do?

To get this in line with the 10/25/50k architecture and considering this is needed, updates the instructions for the 3k and 5k reference architectures with separate Sidekiq nodes, to also copy the secrets files from the application nodes to the Sidekiq nodes.

Related issues

Zendesk (internal-only)

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