document system-level environment variables for file-based pipeline nodes

[shalberd]

Document system-level environment variables for file-based pipeline nodes at level Jupyterlab, KFP or Airflow runtime, or both.
Follow-up to #3227

What changes were proposed in this pull request?

How was this pull request tested?

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[shalberd]

I layed this out conceptually in terms of where the system-level envs apply to, i.e. Jupyterlab or at runtime Airflow and KFP or both (scope).

@harshad16 what should I write about ELYRA_WRITABLE_CONTAINER_DIR? This is only actually used in KFP.

[shalberd]

@lresende Thank you. Now we just need to pull this, I guess, from readthedocs, which is I guess related to #3205 .
Does there exist an elyra-related readthedocs account?
@romeokienzler FYI

[lresende]

#3245 should fix the doc build

[lresende]

https://elyra.readthedocs.io/en/latest/user_guide/env-variables-file-based-nodes.html