Add documentation for query directives

[patrick91]

We only have documentation for schema directives, we should document query directives, here's an example of how they work: https://github.com/strawberry-graphql/strawberry/blob/main/tests/schema/test_directives.py

[patrick91]

We should call them operation directives, to stay consistent with other docs/articles:

• https://www.apollographql.com/docs/apollo-server/schema/directives/#schema-directives-vs-operation-directives
• https://stepzen.com/blog/graphql-directives
• https://css-tricks.com/working-with-built-in-graphql-directives/

[bradleyoesch]

Should this be closed with the merge of https://github.com/strawberry-graphql/strawberry/pull/2728?