Docs: Add $READTHEDOCS_OUTPUT to environment variable reference (#10407)
* Add $READTHEDOCS_OUTPUT to environment variable reference
* Update docs/user/reference/environment-variables.rst
* Update docs/user/reference/environment-variables.rst
Co-authored-by: Manuel Kaufmann <humitos@gmail.com>
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Co-authored-by: Manuel Kaufmann <humitos@gmail.com>