Update Service Insight docs

[jjngx]

Proposed changes

This PR updates Service Insight documentation. We make clear that the functionality is available only for NGINX Plus.

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

[codecov]

Codecov Report

Merging #3684 (193b5c0) into main (b18ac8f) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3684      +/-   ##
==========================================
+ Coverage   52.35%   52.36%   +0.01%     
==========================================
  Files          59       59              
  Lines       16880    16880              
==========================================
+ Hits         8838     8840       +2     
+ Misses       7747     7745       -2     
  Partials      295      295              

see 1 file with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[ADubhlaoich]

@jjngx Approved, but could you change the docs: argument in the frontmatter to "DOCS-1180"? I can't seem to make an edit suggestion for any lines in that block.

[jjngx]

@jjngx Approved, but could you change the docs: argument in the frontmatter to "DOCS-1180"? I can't seem to make an edit suggestion for any lines in that block.

Added the new id arg.