Update README links for building NGINX Ingress Controller images

[ADubhlaoich]

Proposed changes

This commit changes the README links to be the correct, newer link for the document on building NGINX Ingress Controller.

It also makes them the same link: previously, one pointed at the live URL, while the other pointed at the file in the repository.

Checklist

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

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• 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

[jjngx]

👍🏻

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (16af1f6) 51.92% compared to head (a96a3f2) 51.92%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4700   +/-   ##
=======================================
  Coverage   51.92%   51.92%           
=======================================
  Files          59       59           
  Lines       16973    16973           
=======================================
  Hits         8813     8813           
  Misses       7862     7862           
  Partials      298      298           

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