document global envinroment settings

[rudolfix]

Documentation description

dlt uses certain environment variables that are not a part of standard configuration. they are compiled in a separate module https://deploy-preview-1536--dlt-hub-docs.netlify.app/docs/api_reference/common/known_env

we should link to it (maybe?) from our regular documentation - some of the settings are asked for ie. to change the project dirctory or .dt directory.

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