Initial setup of Docusaurus

Created by: amyrlam

👋 I've worked on Docusaurus and talked to @gaearon about setting it up on this repo!

Followed steps on: https://docusaurus.io/docs/en/installation. (Note: Docusaurus sets up website/ at the root level.)

If ok with this, I can move on to https://docusaurus.io/docs/en/site-preparation - in this PR or another one! Let me know!