Github workflow hooks

[ajfriend]

I'd like to set up the docs to:

• build and deploy when I push to master at ajfriend/h3-py
  • so that I can preview at ajfriend.github.io/h3-py
• build and deploy when changes get landed to master on uber/h3-py
  • so that the latest docs get deployed at uber.github.io/h3-py
  • ideally this would only be through PRs, yeah?
• build (but do not deploy**) on PRs to uber/h3-py so that we can confirm that the docs build without errors before landing