Controller JSON Schema

This replaces #175.

- [x] Each schema should reference one or more [examples](https://github.com/flynn/flynn/blob/6553a5fce8bc6637267b45a88b8de67cbec879b4/controller/examples/examples.go#L60-L95) using a custom `examples` keyword. The value should be an array of schema ids referencing example schemas.
- [x] Generate documentation by combining schemas with examples
- [x] Hook up rake task
- [ ] Fill in schema descriptions
- [ ] Fill in / fix schema type descriptions
- [x] Order schemas
- [x] Add expand / collapse toggle for examples
- [x] Add link to docs navigation
- [x] Link to schema JSON
`headers.map { |name, value| "#{name}: #{value}" }.join("\n")`