Add GraphQL queries for State

Query for States is now supported on the GraphQL API too.

• [GraphQL API] Type Query was extended:

  • Field states was added to Query type.
  • Field state was added to Query type.

• [Documentation] Fix documentation of State:

  • The field name is optional.
  • The field description is optional.

• [Documentation] Fix documentation of StateDraft:

  • Add missing field roles used in the Reviews tutorial.