Expand Ref Arch docs with notes on unsupported and more

What does this MR do?

MR updates the main Reference Architectures docs pages with various additional info:

• Various call outs of unsupported designs or services for better clarity
• Some general guidance for Upgrades and scaling
• Expands 'Before You Start' section to call out Professional Services as well as to considered longer term maintenance plans.
• Call outs for Consul, Redis Sentinel and Praefect quorum requirements
• Removes line about adjusting Gitaly IOPS, this is still yet to be confirmed.

Related issues

Closes gitlab-org/quality/reference-architectures#175

Author's checklist

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Reviewer's checklist

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