Add initial guidance over smaller 3k RA tweaks

What does this MR do?

MR adds initial supported guidance for modifying the 3k Reference Architecture for smaller numbers of users where performance isn't as much a concern.

Also brings in some more formatting from the 10k and adjust Redis Sentinel to use non burstable instance types.

Related issues

Closes gitlab-org/quality/reference-architectures#51 gitlab-org/quality/reference-architectures#39

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