Add high level doc for data plane config

[sjberman]

Problem: As a user, I want to know how I can configure global settings for nginx.

Solution: Add a doc that talks about how global data plane config can be set.

Closes #2246
Closes #2119

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

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes,
please add a brief note that summarizes the change.

NONE

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.83%. Comparing base (2ac3239) to head (c12fbb7).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2378      +/-   ##
==========================================
- Coverage   88.86%   88.83%   -0.03%     
==========================================
  Files         100      100              
  Lines        7515     7515              
  Branches       50       50              
==========================================
- Hits         6678     6676       -2     
- Misses        781      782       +1     
- Partials       56       57       +1     

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[ADubhlaoich]

LGTM. A few suggestions, but norhing breaking.