chore: import upstream snapshot with attribution

docs/source/env_var.rst

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+Environment Variables
+=====================
+
+Global Configurations
+---------------------
+* ``DGLDEFAULTDIR``:
+    * Values: String (default=``"${HOME}/.dgl"``)
+    * The directory to save the DGL configuration files.
+
+* ``DGL_LOG_DEBUG``:
+    * Values: Set to ``"1"`` to enable debug level logging for DGL
+    * Enable debug level logging for DGL
+
+Backend Options
+---------------
+* ``DGLBACKEND``:
+    * Values: String (default='pytorch')
+    * The backend deep learning framework for DGL.
+    * Choices:
+        * 'pytorch': use PyTorch as the backend implementation.        
+        * 'tensorflow': use Apache TensorFlow as the backend implementation.
+        * 'mxnet': use Apache MXNet as the backend implementation.
+
+Data Repository
+---------------
+* ``DGL_REPO``:
+    * Values: String (default='https://data.dgl.ai/')
+    * The repository url to be used for DGL datasets and pre-trained models.
+    * Suggested values:
+        * 'https://data.dgl.ai/': DGL repo for Global Region.
+        * 'https://dgl-data.s3.cn-north-1.amazonaws.com.cn/': DGL repo for Mainland China
+* ``DGL_DOWNLOAD_DIR``:
+    * Values: String (default=``"${HOME}/.dgl"``)
+    * The local directory to cache the downloaded data.