Add Cosmic JS Starter (#5916)

* Cosmic JS Gatsby Blog article

* Cosmic JS source plugin link

* fix: miscellaneous edits

- removed an intro to Gatsby, which isn't necessary on the Gatsby blog
- minor cleanup for clarity and grammar
- removed some language that can be problematic for beginners (e.g. "obviously")
- added links to relevant Gatsby concepts and APIs

* rm gatsby image, add config notes

* add canonicalLink

* Update index.md

* cosmicjs starter

* cosmicjs starter copy

* Delete package-lock.json

docs/docs/gatsby-starters.md

@@ -55,6 +55,14 @@ gatsby new my-blog https://github.com/gatsbyjs/gatsby-starter-blog#v2
 
 ## Community starters
 
+- [gatsby-starter-cosmicjs](https://github.com/cosmicjs/gatsby-starter)
+  [(demo)](https://cosmicjs-gatsby-starter.netlify.com)
+
+  Features:
+
+  - Same as `gatsby-starter-default` with dynamic content from [Cosmic JS](https://cosmicjs.com)
+  - Install and set up CMS in seconds [via the command line](https://github.com/cosmicjs/gatsby-starter)
+
 - [gatsby-starter-blog-no-styles](https://github.com/noahg/gatsby-starter-blog-no-styles)
   [(demo)](http://capricious-spring.surge.sh/)