Update CONTRIBUTING.md

[jputrino]

Proposed changes

Clarifies the section of the contribution guide that covers docs not developed in this repo:

• adds nginx/nginx.org repo
• adds nginx/nginx-agent repo
• removes nginx/nginx-gateway-fabric repo because the NGF docs now reside here
• adds information to each repo in the list that indicates what it is the source for

Checklist

Before merging a pull request, run through this checklist and mark each as complete.

• I have read the contributing guidelines
• I have signed the F5 Contributor License Agreement (CLA)
• I have ensured that documentation content adheres to the style guide
• If the change involves potentially sensitive changes, I have assessed the possible impact
• 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
• I have updated any relevant documentation (README.md and CHANGELOG.md)
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Potentially sensitive changes include anything involving code, personally identify information (PII), live URLs or significant amounts of new or revised documentation.

Please refer to our style guide for guidance about placeholder content.

[mjang]

Makes sense.

[github-actions]

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