Add `ZE_FLAT_DEVICE_HIERARCHY` and `ZE_AFFINITY_MASK` to documentation on environment variables

[ndgrigorian]

Since ZE_FLAT_DEVICE_HIERARCHY and ZE_AFFINITY_MASK direct impacts the (now-supported) composite devices, it should be mentioned in
https://intelpython.github.io/dpctl/latest/user_guides/environment_variables.html

see:
https://www.intel.com/content/www/us/en/developer/articles/technical/flattening-gpu-tile-hierarchy.html

[ndgrigorian]

Closed by #2137