Automate initial setup for new contributeres

[ghost]

• Introduces the yarn setup script to easily clone required git repositories for building Ember API documentation locally.

if you do not have cloned repositories

$ yarn setup
yarn run v1.17.3
$ node -r esm setup-repositories.js
Running 'git clone git@github.com:emberjs/ember.js.git' in /Users/eli/floss/ember/contributing
Running 'git clone git@github.com:emberjs/data.git' in /Users/eli/floss/ember/contributing
Running 'git clone git@github.com:ember-learn/ember-api-docs.git' in /Users/eli/floss/ember/contributing
✨  Done in 6.64s.

if you already have cloned versions

$ yarn setup
yarn run v1.17.3
$ node -r esm setup-repositories.js
ember.js is already cloned to /Users/eli/floss/ember/contributing/ember.js
data is already cloned to /Users/eli/floss/ember/contributing/data
ember-api-docs is already cloned to /Users/eli/floss/ember/contributing/ember-api-docs
✨  Done in 0.42s.

I can update the README with this once we reach consensus / make any needed changes.

[sivakumar-kailasam]

Thanks for the PR @efx.

Usually users have the other repos cloned on their machines before they clone this repo. Maybe instead of cloning it for them how about we add extra args to the generator script to pass the paths to the project directories instead?

[ghost]

@sivakumar-kailasam ah, that sounds reasonable. I cannot guarantee time to work on that now but will consider it.