docs: Add docs for basic auth to airflow stable rest api

[chrishronek]

Summary

These docs will help your users utilize the Airflow Stable REST API for Airflow instances running on plural.sh

Test Plan

I've completed these steps on my plural.sh Airflow instance successfully.

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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Not sure if much of the checklist applies since it's just a doc add for an existing app - let me know if you want me to change anything 🍻

[michaeljguarino]

awesome, thanks for the contribution!