[Draft] docs: add missing prerequisite for installation

[dkleinF5]

Proposed changes

Checklist

Before sharing this pull request, I completed the following checklist:

• I read the Contributing guidelines
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• My content changes adhere to the F5 NGINX Documentation style guide
• If my changes involve potentially sensitive information¹, I have assessed the possible impact
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Footnotes

1. Potentially sensitive information includes personally identify information (PII), authentication credentials, and live URLs. Refer to the style guide for guidance about placeholder content. ↩

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[dkleinF5]

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