Adding a quick link to the source code / schema of example at docs

Hi team,

I am trying to learn `GraphQL` by reading [the docs](https://graphql.org/).

While I personally found the [interactive examples](https://graphql.org/learn/queries/#fields) quite useful, I was also quite curious about the actual schema / data behind the interactive examples.

As discussed in #728, I believe [the schema and data can be found here at `graphql.github.io/site/_core/swapiSchema.js`](https://github.com/graphql/graphql.github.io/blob/739a1c913fa4a88ac7c64c93ce68fc64ec0cbd96/site/_core/swapiSchema.js#L21)

However, I wonder if we could make the documentation more user-friendly by adding a quick link at the top of the documentation page, say, something like `You can find the schema and data behind these interactive examples here`.

Anyway, thanks again for your tremendous efforts in maintaining the library and docs!