docs: Scaling horizontally on kubernetesexecutor (airflow)

[chrishronek]

Summary

Just a simple docs add to help plural users to scale their Airflow workloads when using the KubernetesExecutor

Test Plan

These are steps that I've taken in my own Plural Airflow application and they worked wonders for me.

Checklist

Deleted the checklist because it didn't really apply to a docs addition

If I missed anything, please let me know 🍻

[chrishronek]

is there a reason the custom node doc was removed?

I decided to combine it with this doc because it felt like they were the same topic