We're testing a workflow for users to start with Gatsby Cloud, and then pull the sites down locally to continue iterating upon and exploring the starter. This change adds a quickstart link for our only official starter that requires no environment variables or additional configurations.

- Including UTM so we can see the degree to which users actually try this flow.
- Further clarified the outcome of the cloud-first onboarding flow
- Further differentiated the "try on cloud" vs "try locally" sections

… repo and this package directory readme