update readme

[slsriehl]

The following changes conclude the repo-specific documentation work originally scoped to support this release:

• move title above deploy button
• clarify that deploy button & auto repo creation is the way to go
• add links to community and graphql carts & checkout
• directions on accessing the playground

[vercel[bot]]

@slsriehl is attempting to deploy a commit to the BigCommerce Platform Team on Vercel.

A member of the Team first needs to authorize it.

[becomevocal]

👍 👍