Add production documentation link for OpenTelemetry

[ADubhlaoich]

Proposed changes

This commit replaces the example text for OpenTelemetry with the production website URL, creating one single source of truth for instruction in a place more easily discoverable to users.

It is follow-up work from the documentation PR:

nginx/documentation#824

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

[ADubhlaoich]

This PR should not be merged before the documentation one.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.95%. Comparing base (74fb6af) to head (5bcbf3a).
Report is 2 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@ ## main #8041 +/- ## ========================================== - Coverage 52.96% 52.95% -0.01%  ========================================== Files 90 90 Lines 21729 21729 ========================================== - Hits 11508 11506 -2  - Misses 9744 9746 +2  Partials 477 477 

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