Update example config for netlify-cms (#5405)

Added the public_folder attribute to make the assets accessible.

Also added the body field to the collection, so the html attribute of markdownRemark gets filled properly. Figuring out that the field has to be named "body" was a bit cumbersome.

https://www.netlifycms.org/docs/configuration-options/#fields

docs/docs/netlify-cms.md

@@ -63,6 +63,7 @@ backend:
   name: test-repo
 
 media_folder: static/assets
+public_folder: assets
 
 collections:
   - name: blog
@@ -73,6 +74,7 @@ collections:
       - { name: path, label: Path }
       - { name: date, label: Date, widget: date }
       - { name: title, label: Title }
+      - { name: body, label: Body, widget: markdown }
 ```
 
 Then in your terminal run `gatsby develop` to start the Gatsby development server. Once the server