fix(docs): added Cosmic to headless-cms page (#11106)

Added Cosmic to the Headless CMS page

Author: tonyspiro

docs/docs/headless-cms.md

@@ -17,6 +17,7 @@ The guides in this section will walk through the process of setting up content s
 
 Other CMS systems you can connect to include:
 
+- [Cosmic JS](https://www.gatsbyjs.org/packages/gatsby-source-cosmicjs)
 - [ButterCMS](https://www.gatsbyjs.org/packages/gatsby-source-buttercms)
 - [Shopify](https://www.gatsbyjs.org/packages/gatsby-source-shopify)
 - [Contentstack](https://www.gatsbyjs.org/packages/gatsby-source-contentstack)