docs: Add a guide for running airflow tasks on custom nodes

[chrishronek]

Summary

Shows users how to point Airflow tasks at custom node groups

Test Plan

Currently running this in my plural.sh hosted 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

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A lot of the checklist items don't feel applicable with this just being a doc add. Let me know if I missed anything! 🍻

[michaeljguarino]

We should add a mention that this step is optional only if you want to ensure at least 3 spot nodes are present at any time for latency