Update v3 NGINX Agent documentation link

[aknot242]

Proposed changes

Updates the README.md Agent v3 link to point to the correct location in the N1C docs. Fixes #1491

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.01%. Comparing base (02f4c36) to head (305056f).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1492      +/-   ##
==========================================
- Coverage   85.13%   85.01%   -0.12%     
==========================================
  Files         102      102              
  Lines       13217    13217              
==========================================
- Hits        11252    11237      -15     
- Misses       1475     1492      +17     
+ Partials      490      488       -2     

see 5 files with indirect coverage changes

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[aknot242]

I don't have merge rights in this projects. Would someone be able to perform the merge?