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

• No images hosted from dockerhub
• Are dashboards present to understand the health of the application. There must be at least 1 of these
  • all databases should have dashboards
  • ideally also have at least cpu/mem utilization dashboards for webserver tier of the app
  • you can use plural from-grafana to convert a grafana dashboard found via google to our CRD
• Are scaling runbooks present
  • all databases must have scaling runbooks
  • you can use the charts in pluralsh/module-library to accelerate this
• do you need to add config overlays?
  • inputing secrets
  • configuring autoscaling
• 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
• All major clouds must be supported
  • Azure
  • AWS
  • GCP

---

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

[michaeljguarino]

there's also an annoying createPluralSecret: true config variable needed here

[chrishronek]

@michaeljguarino - just added it (I think). Can you confirm that I got it right?