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

[SetTrend]

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.

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.