[www] add gatsby-orga starter (#3908)

Author: xiaoxinghu

docs/docs/gatsby-starters.md

@@ -444,3 +444,10 @@ Community:
   * auto-generated sizes and types (png, webp) for background and hero images
   * favicons generator
   * webfonts with [webfontloader](https://github.com/typekit/webfontloader)
+  
+* [gatsby-orga](https://github.com/xiaoxinghu/gatsby-orga)
+  [(demo)](https://xiaoxinghu.github.io/gatsby-orga/)
+  
+  Features:
+  
+  * Parses [org-mode](http://orgmode.org) files with [Orga](https://github.com/xiaoxinghu/orgajs).