Hey Anton, we are currently working on having a Demo with the gatsby-source-prismic plugin, so it isn’t released just yet. But we’ll give updates when it’s ready.
About the Providing JSON schemas part, you’re right that maybe it’s isn’t super clear there.
You just need to create a directory where they tell you to and create one file per Custom type in your repo, then in those files you just need to paste the JSON of each Custom type, you find these in your repository > Custom types, open any Custom type and click on the JSON editor tab, copy that JSON and paste in your file, and so on!