Document environment variables (#2552)

* Document envvars

Signed-off-by: Kirthi Shankar Sivamani <[email protected]>

* Add remaining envvars

Signed-off-by: Kirthi Shankar Sivamani <[email protected]>

* More missing ones

Signed-off-by: Kirthi Shankar Sivamani <[email protected]>

* Update docs/envvars.rst

Co-authored-by: Paweł Gadziński <[email protected]>
Signed-off-by: Kirthi Shankar Sivamani <[email protected]>

* Update docs/envvars.rst

Signed-off-by: Kirthi Shankar Sivamani <[email protected]>

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Signed-off-by: Kirthi Shankar Sivamani <[email protected]>
Co-authored-by: Paweł Gadziński <[email protected]>

Author: ksivaman