[Improvement request]: Segregate GraphGL standard implementation features from GraphGL extensions

The current documentation doesn't clearly differentiate between the core GraphGL standard and implementor specific extensions (e.g. Relay).

See [this discussion on **graphql/graphql-spec**](https://github.com/graphql/graphql-spec/discussions/1083).

Please, add a documentation page depicting …
(a) the full scope of the core GraphGL standard, and
(b) a hyperlink to different extension specs, optionally including a brief overview of the corresponding extension's features.

Start by reviewing the current GraphQL documentation and the linked graphql/graphql-spec discussion. The completed page should define the full scope of the core GraphQL standard and link to extension specifications, with brief feature overviews where appropriate.

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