GraphQL HTML documentation

[simon-wacker]

You can run make serve, open localhost:4000 and localhost:4001 to view the GraphiQL IDE for ./apis/database.graphql and ./apis/metabase.graphql, and use its documentation browser. Note that GraphQL Playground will become part of GraphiQL IDE.

Alternatively, we could try to get DociQL working. It's prettier than GraphiQL but badly maintained.