DGL document and tutorial folder

Requirements

• sphinx
• sphinx-gallery
• Both pytorch and mxnet installed.

Build documents

First, clean up existing files:

./clean.sh

Then build:

make html

Note: due to the backend loading issue, it actually takes 2 rounds to build:

1. build tutorials that uses MXNet as backend
2. build tutorials that uses PyTorch as backend

Render locally

cd build/html
python3 -m http.server 8000