feat: use yarn

[MichaelGoberling]

• Update readme docs to use yarn instead of npm
• Update yarn to 4.4.0

[YevSent]

If you take a look at package.json, it already has commands that are wrappers around yarn. Do you think it makes sense to expose yarn commands separately and use them instead of the commands specified in package.json?

[MichaelGoberling]

Yarn does run the scripts from package.json

For instance yarn dev runs the "dev": ... script, yarn build runs "build": ... and so on

You can confirm by running yarn dev --help, first line will be gatsby develop which is the script command