Add documentation for using context values with `@authorization`

[Liam-Doodson]

Add docs for this feature. An example of it's usage can be found in our tests here: https://github.com/neo4j/graphql/blob/38d26bd0ecb589a187a6f8fea4cc59b3c4242489/packages/graphql/tests/integration/directives/authorization/object-path.int.test.ts#L96

However, it looks like we only document how to use jwt values in the docs: https://neo4j.com/docs/graphql/current/security/authorization/#_filtering_rules