chore(starters): add gatsby-contentful-starter (#18658)
chore(starters): add gatsby-contentful-starter (#18658)

* chore(starters): add gatsby-contentful-starter

I have created a starter utilizing Contentful as the CMS, and deployed to Netlify. If there are any issues with my PR, just let me know.

* Update starters.yml

* chore: format

gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby