[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.