Adds ButterCMS to source list of CMS systems (#9784)

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One of two changes for #9757

Second change will be a complete guide for ButterCMS. I'll open a new PR for that.

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:
 
+- [ButterCMS](https://www.gatsbyjs.org/packages/gatsby-source-buttercms/?=gatsby-source-)
 - [Shopify](https://www.gatsbyjs.org/packages/gatsby-source-shopify/?=gatsby-source-)
 - [Contentstack](https://www.gatsbyjs.org/packages/gatsby-source-contentstack/?=gatsby-source-)
 - [Ghost](https://www.gatsbyjs.org/packages/gatsby-source-ghost/?=gatsby-source-)