Add configuration overview

[oCHRISo]

Proposed changes

Adding configuration section to docs
Providing overview of ways to configure NGINX Agent (conf file, CLI and Environment vars)

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13

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[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (main@fda5ae8). Click here to learn what that means.
Patch has no changes to coverable lines.

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Additional details and impacted files

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  Files           ?      113           
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  Hits            ?     8529           
  Misses          ?     3613           
  Partials        ?      572           

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