Configure Netlify site

[coalkettler]

We currently have this repo building on Netlify: https://app.netlify.com/sites/geotrellis-docs/overview

We should move the site and build configuration into a netlify.toml file and commit it to version control.

[CloudNiner]

Current build command is npm install -g yarn && ./scripts/cibuild. I'm sure netlify.toml specifies a way to use yarn instead.