docs: add KongServiceFacade docs and examples

[czeslavo]

What this PR does / why we need it:

Adds a new FEATURE_GATES.md section "Using KongServiceFacade" explaining the basics of KongServiceFacade. It also references a new example manifest that demonstrates complete usage of KongServiceFacade to set a different set of plugins for the same Kubernetes Service.

The instructions in the example manifest depend on Kong/charts#963 which adds RBACs for KongServiceFacade.

PR also adds an integration test that verifies the new example manifest to make sure it doesn't get outdated.

Which issue this PR fixes:

Closes #5152.

Special notes for your reviewer:

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6470f94) 75.9% compared to head (5aa80f5) 75.8%.
Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #5298     +/-   ##
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- Coverage   75.9%   75.8%   -0.1%     
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  Files        170     170             
  Lines      19458   19447     -11     
=======================================
- Hits       14769   14756     -13     
- Misses      3858    3860      +2     
  Partials     831     831             

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[mflendrich]

Looking good 👍

Only the kustomize command comment is blocking (as the original URL did not work for me with a spurious kustomize error).

[czeslavo]

@mheap @mflendrich Thanks for your comments, PTAL again 👍