use yarn instead of npm

[kevinbarabash]

npm has changed the format of package-lock.json between v5 and v6. The yarn.lock format is intended to be stable so even if the organization of node_modules changes between versions, yarn.lock should remain unchanged. Also, yarn install doesn't update its lock file whereas you have to pass npm the --no-save option if you want to prevent changes to `package-lock.json. The downside of this change is that people will have to install yet another package manager.

[ylemkimon]

In #1484, Docusaurus recommends using yarn, too.