Move cypher directive content to its own page

[johnymontana]

The cypher directive feature is powerful enough that it should have its own page, with examples showing:

• [ ] computed scalar fields
• [ ] computed object and array fields
• [ ] custom top level query fields
• [ ] custom top level mutation fields
• [ ] use cases for the above
• [ ] using cypher / GraphQL field parameters
• [ ] using context values

Additionally, consider adding a guide to show best practice usage of above (and what not to do)