Add an example to the sourceNodes docs (#8585)
Add an example to the sourceNodes docs (#8585)

* Add an example to the sourceNodes docs

* Formatting

* Pass a unique value in to createNodeId

* Remove stray const

gatsbyjs/gatsby,0x80/gatsby,gatsbyjs/gatsby,mingaldrichgan/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,0x80/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,mingaldrichgan/gatsby,gatsbyjs/gatsby,0x80/gatsby,mingaldrichgan/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby