Updating English Docs

1. modify rsts under docs/source/tutorial_doc
2. run ci/build_doc.sh or cd docs && make clean && make html to build english under build/html
3. please update Chinese version accordingly.

Updating Chinese Docs

1. run cd docs && make gettext && sphinx-intl update -p ./build/gettext -l zh_CN This generates the intermidiate products under source/locale/zh_CN/LC_MESSAGES
2. find the parts you modified in the English version.
3. run sphinx-build -D language=zh_CN -b html ./source build/html/zh-CN to build the chinese version under build/html/zh-CN