Add Cloud Native Hybrid instructions on 50k users RA page

What does this MR do?

The MR adds 50k hybrid environment guidance based on the performance testing in gitlab-org/quality/reference-architectures#24 (comment 608540501).

Related issues

• gitlab-org/quality/reference-architectures#24 (closed)
• gitlab-org/charts/gitlab!2063 (merged)

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have Developer permissions or higher:
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Review checklist

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1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

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• Clear typos, like this is a typpo.
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3. Maintainer

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