docs: re-add agent config doc

[JTorreG]

Proposed changes

Add an agent doc that was mistakenly removed

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. ↩

[github-actions]

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[nginx-seanmoloney]

Thank you!

[ADubhlaoich]

LGTM, small edits.

Really hate the indented list sequence for instructions but I understand it's a system issue that would involve extra effort to fix.