Add a live reference page for GraphQL

[m-allanson]

The aim is to have a reference page with an embedded GraphiQL instance running with the current gatsbyjs.org data. Example queries can then be demo'd with real inputs and results.

• create a standalone GraphiQL instance from gatsby/www
• host the instance on a public url
• configure the instance to automatically update with new Gatsby releases
• Write new GraphQL reference page with live example queries

See comment in #3557 (comment)

[m-allanson]

Adding this to track progress - I've had the first step working locally for some time. For step two I was planning to host on Heroku (as that's super simple to hook up to git) but I've had trouble getting it fired up inside Heroku's startup time limits. The next step is to set up the hosting somewhere else instead.

[m-allanson]

Done 🎉 https://www.gatsbyjs.org/docs/graphql-reference/