Supported (GQL) data types are not documented

[AndrewSisley]

We currently have no section that notes which GQL types Defra supports. This is particularly important for highlighting standard GQL types that we do not support, for example NonNull types.

The exact types that we do support can be found in semi-human readable form here: https://github.com/sourcenetwork/defradb/blob/develop/request/graphql/schema/descriptions.go (fieldKindToGQLType).

[orpheuslummis]

I'm not sure where to include that information