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
• My branch adheres to the Git conventions
• My content changes adhere to the F5 NGINX Documentation style guide
• If my changes involve potentially sensitive information¹, I have assessed the possible impact
• I have waited to ensure my changes pass tests, and addressed any discovered issues

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]

I have hereby read the F5 CLA and agree to its terms

[JTorreG]

There is a TODO label

[ADubhlaoich]

Generally LGTM: approval pending feedback.

Please run the linting tools on this branch, which can be executed automatically with pre-commit.

[ADubhlaoich]

Some more minor changes for content flow and formatting.