docs: Add a guide for configuring the kubernetes executor

[chrishronek]

Summary

This should show your customers who would like to use the Airflow KubernetesExecutor a good starting point to do so.

Test Plan

I'm running the current config in my plural.sh Airflow and it's working great

Checklist

• [x] No images hosted from dockerhub
• [x] Are dashboards present to understand the health of the application. There must be at least 1 of these
  • [x] all databases should have dashboards
  • [x] ideally also have at least cpu/mem utilization dashboards for webserver tier of the app
  • [x] you can use plural from-grafana to convert a grafana dashboard found via google to our CRD
• [x] Are scaling runbooks present
  • [x] all databases must have scaling runbooks
  • [x] you can use the charts in pluralsh/module-library to accelerate this
• [x] do you need to add config overlays?
  • [x] inputing secrets
  • [x] configuring autoscaling
• [x] If there’s a web-facing component to the app, we need to support OIDC authentication and setting up private networks if no authentication option is viable
• [x] All major clouds must be supported
  • [x] Azure
  • [x] AWS
  • [x] GCP

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Checklist feels not applicable mostly becuase it's a simple doc add for an existing application. Let me know if I missed anything 🍻