[Docs] Write Page on how to build Custom Extractors for Airflow

[TohnJhomas]

One of the strengths of airflow is the way it democratizes orchestration through operators, and as a result many users have custom or less-popular operators as key components in their pipelines. In order to get lineage out of these tasks, they'll have to also write an extractor, a process that's currently poorly documented and therefore slightly opaque.

This document should initially focus on being a tutorial rather than a comprehensive technical reference, and we can flesh out technical details as gaps become apparent.

Resources:

• Airflow README

[denimalpaca]

We can close this