Add more guidance for external Redis services

What does this MR do?

Adds some more guidance for external Redis service setup in the documentation. Specifically around the separated Cache and Persistent instances.

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
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  • Style Guide.
• If you're adding or changing the main heading of the page (H1), ensure that the product tier badge is added.
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• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

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• If the content requires it, ensure the information is reviewed by a subject matter expert.
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• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.