Automate documentation website update

[terrytangyuan]

Currently we have to manually update the website in gh-pages branch, which includes:

• Delete everything except the .git and the .nojekyll (reference)
• Paste the content of _build/html/* after running cd doc; make html on the master branch.

This process should be automated.

[terrytangyuan]

Using Github Actions to publish new websites might be a good approach as well: https://github.com/sdras/awesome-actions#github-pages