add documentation for group by directive

[angrykoala]

This is the documentation for the PR neo4j/graphql#7061

[a-alle]

Left a couple of suggestions that make sense together, to create a more coherent flow. Don't feel too strongly about it so feel free to ignore but in my opinion it reads better.

Also, should we mention that relationship fields and non-primitive types cannot be annotated with this directive?

[a-alle]

should this have the empty paranthesis?

[a-alle]

Suggested change

	The `groupBy` field has one argument `fields`, that defines the field to group by.
	The `groupBy` field has a single, mandatory argument `fields` which defines the field to group the results by.
	The type of the `groupBy` field is a list of results compliant to the Connection Spec.

[a-alle]

Suggested change

The field allows you to query movies grouped by the `year` field.

[a-alle]

Suggested change

	In this case `year` is the only option.
	The following example shows how to query movies grouped by the `year` field.
	In this case, `year` is the only field the results can be group by.

[a-alle]

Suggested change

	The `groupBy` field returns a list of results and each result contains a list of `edges`, with all results grouped by the `year` field.
	The result of this query is a list of `edges` wherein all results share the same value of the `year` field.

[a-alle]

Suggested change

For example:

[a-alle]

Suggested change

Note that the `year` field is repeated on every node of the `edges` from each `groupBy` result.

[a-alle]

Suggested change

	Nodes can be grouped by multiple fields:
	Nodes can be grouped by multiple fields.
	Consider the following type definition:
	[source, graphql, indent=0]
	----
	type Movie @node {
	title: String!
	year: Int! @groupBy
	rating: Int! @groupBy
	}
	----
	You can now use both `year` and `rating` fields in the argument of the `groupBy` field.

[a-alle]

One more thing: I feel like this feature is too buried, it's 3 levels deep inside a page with a sort of unrelated title. I'd never think to search for this feature there. Is there any place in the docs that's more visible where we can link this section? Maybe in queries?