Add 'environmental_variables.md'

docs/server_configuration/environmental_variables.md

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+# Environmental vaiables
+
+`Inference` behavior can be controlled by set of environmental variables. All environmental variables are listed in [inference/core/env.py](inference/core/env.py)
+
+Below is a list of some environmental values that require more in-depth explanation.
+
+Environmental variable                     | Default                                                                  | Description
+------------------------------------------ | ------------------------------------------------------------------------ | -----------
+ONNXRUNTIME_EXECUTION_PROVIDERS            | "[CUDAExecutionProvider,OpenVINOExecutionProvider,CPUExecutionProvider]" | List of execution providers in priority order, warning message will be displayed if provider is not supported on user platform