feat(user-guide): add inputOutput, connectors and scripting section

- add section for input/output variable mapping to user guide
- add section for scripting to user guide
- add section for connectors to user guide
- add camunda:inputOutput and child elements to api-reference
- add camunda:connector to api-reference
- add camunda:resource to api-reference

related to #CAM-2326, CAM-2329, #CAM-2357